This is a case of ellipsis, where few words are 'implied' rather than explicitly stated. In such a scenario, the words that are omitted necessarily have to be the same as the words used in the first half of the sentence. So, A in its current form would read:

To develop more accurate population forecasts, demographers have to know a great deal more than (demographers have to know) now about the social and economic determinants of fertility.

This is clearly not correct, since the actual sentence needs to be:

To develop more accurate population forecasts, demographers have to know a great deal more than (demographers know/do) now about the social and economic determinants of fertility.

This is what D says and is hence correct.

If A is kept the way it is, we have a problem of illogical comparison. Ask yourself what is being compared?

Answer A compares what demographer have to now with now, which is illogical. Simply put the comparison needs to be a full one, as much as possible, so there is no problem of lack of clarity. What should be compared is the demographer's knowledge in the hypothetical future ("would have to know more...") with what they actually know now. By omitting the subject and the verb of the first half of the comparison, the sentence becomes nonsensical.

Here is a simple example of how that extra bit of information can alter the sentence completely.

I like Julie more that Brenda.

Here is it clear that I compare my like of Julie with my like of Brenda.

However, let us change the sentence to: I like Julie more than Brenda does.

Now the comparison is between Brenda and I and how much we like Julie, respectively.

So in a comparison, be careful and ask yourself whether the comparison is correct (the two things that should be compared are actually compared) and whether the comparison is logical.

Isaac Betan wrote:
I like Julie more that Brenda.

Here is it clear that I compare my like of Julie with my like of Brenda.

Hi!! don't think this is correct. This is actually an ambiguous statement:

I like Julie more that Brenda.

This can be interpreted as either of the following:

1. I like Julie more that I like Brenda.
2. I like Julie more that Brenda likes Julie.

To overcome this ambiguity, we need to phrase the sentence as:
1. I like Julie more that I like Brenda.
2. I like Julie more that Brenda does (likes Julie).

Thanks.

mohish wrote:

Isaac Betan wrote:
I like Julie more that Brenda.

Here is it clear that I compare my like of Julie with my like of Brenda.

Hi!! don't think this is correct. This is actually an ambiguous statement:

I like Julie more that Brenda.

This can be interpreted as either of the following:

1. I like Julie more that I like Brenda.
2. I like Julie more that Brenda likes Julie.

To overcome this ambiguity, we need to phrase the sentence as:
1. I like Julie more that I like Brenda.
2. I like Julie more that Brenda does (likes Julie).

Thanks.

Maybe you missed typed the same "THAT" every time.

THAT should be "THAN".

And yes, your concept in this regard is correct.

@ Isaac Betan : How would we justify this comparison question :

In no other historical sighting did Halley's Comet cause such a worldwide sensation as did its return in 1910-1911.

(A) did its return in 1910-1911
(B) had its 1910-1911 return
(C) in its return of 1910-1911
(D) its return of 1910-1911 did
(E) its return in 1910-1911

Could you explain why in A comparison is between "the comet and its return" ?

I think the comparision ( comet causing sensation ) is between in no other historical date and in 1910-1911

IMO A

In reply to gmat_perfect,

yes, I meant 'than' rather than 'that'.

As for the Halley's comet question, the answer is not A because of the faulty comparison embedded within the parallel structure. In answer choice A, its return is compared with the historical sighting/time.
Because the verb did is written before Halley's comet and again before its return, we can see what the two items being compared are (the comet and the return). What should be compared are the two times when the comet appeared and caused a sensation.

Therefore, there is the hint via the verb did, but there is another hint: When there are prepositional phrases on two sides of the comparison equation, look at the actual prepositions. In this case in (in no other historical sighting). Therefore in no other historical sighting should be compared to in its return not just its return, so that two times are being compared and not a time with an event.