This question should be in the Problem Solving section.

Beyond that, the question is unclear at best. Is Harry the reseller? Or, is Harry the final purchaser?

The formula to use is very clear:
S = P + .4S
or, .6S = P

However, S or P could be $150 depending on how we read the question. I assumed, wrongly, that the 'tense' determined the value.

Harry "bought" AND Selling price "was" indicate that Harry bought the desk as the selling price because both happened in the past(basically, he is NOT the reseller because the selling price would be spoken about in the present or future).

So, (.6)$150 = P
P = 90, Gross Profit 150-90 = 60 or B.

But, your official answer of 100 or E. means that Harry is in fact the reseller (which is entirely unclear from the question).
.6S = $150
S = $250
$250 - $150 = $100 Gross Profit

60 or B is the correct Choice..

Heres how I did it -

PP = 150$
SP= PP + Markup
SP= 150 + (40/100)[sp]
SP=150+0.4SP
0.6SP = 150

SP = 150/0.6 = 250

So, Gross profit =SP - PP = 250-150 =100 $

So, E is correct.

Issue with Clarity -

In number of questions on the practice GMAT prep the questions are not clear.
I am facing the same problem, donot know if that will be the case in actual exam.

what does markup means iin the above question ? That makes the whole question unclear. Can we get such types of words in an actual Gmat question ?