Find the grammatically incorrect part of the sentence, if any.
A)  The creature on Mars, if any
B) are bound to be different from us
C)  not only in shape but also in size
D) because of different gravitational conditions.

The critter on Mars, if any, are bound to be different from us not only in shape but also in size because of different gravitational conditions. 
In this judgment , the mistake can be fluently spotted in the first part of the judgment . The helping verb used in this sentence is “ are ”. “ Are ” is used with a form of plural noun. The noun given is singular, that is, critter. It has to be in the form of plural noun to be used with verb “ are ”. therefore, the correct noun should be ‘ brutes ’. 
The correct judgment should read as The brutes on Mars, if any, are bound to be different from us not only in shape but also in size because of different gravitational conditions. 
thus, the answer to the given question is option a since the grammatical error is in the part of judgment under optiona. 
The rest of the options( b, c, and d) have correct alphabet in them which is why they're wrong answers. 
Note To identify the grammatical error, it's helpful if you precisely read the whole judgment . also check each part precisely.