Hi Abhishek009,

You are correct. In addition to the rules that you listed, there is an additional Parallelism rule involving the words "exercise" and "associate."

GMAT assassins aren't born, they're made,
Rich

I used to think if sb. do sth., they will do sth.
Here the case is: if they do, they do.

Both of these constructions can be correct, depending on the meaning. Consider:

If I study hard for the GMAT, I will do well.

Here, a specific action in the present tense "study" will lead to an action in the future: "will do well."

When we're talking about things that are generally true / generally occurring, we can use present tense for both:

If students study hard for the GMAT, they do well.

Here, we're not talking about one cause leading to one event, but about events that are generally true for an entire group of people. As such, we use present tense for both.

It would not be incorrect in your example sentence to say "if calves exercise and associate with other calves, they will require less medication and gain weight quicker." The future tense would imply that none of these calves had done the requiring or gaining yet. But it makes slightly more sense to use the present tense, since there are presumably calves right now who are currently requiring less medication and gaining weight.

So, instead of looking for what you want the correct answer to say (if present, then future), just eliminate the answers that are incorrect:

(A) associated with other calves, they will require less medication and gain weight quicker than do
"associated" must be parallel to "exercise"

(B) associated with other calves, they require less medication and gain weight quicker than
same issue here

(C) associate with other calves, they required less medication and will gain weight quicker than do
This inverts cause/effect. The requiring of medication should be the result o exercising and associating. Also, "require" and "gain" should be parallel.

(D) associate with other calves, they have required less medication and will gain weight more quickly than do
Same issue as in C.

(E) associate with other calves, they require less medication and gain weight more quickly than
"exercise" and "associate" are parallel; "require" and "gain" are parallel. Correct!

Here is another related example:
https://www.beatthegmat.com/a-simple-tru ... tml#637051