The correct answer is "fewest" so the sentence comples as follows:
Of all the students in that group, Malcolm usually makes the Blank Space fewest mistakes.
Comparative and superlative adjectives are used when comparing two or more things.
In this case, the options "more fewer" and "most fewest" are incorrect as we cannot use double comparative or superlative. The words more and most are not used when the comparative or superlative adjective is short and we add "er" or "est".
The option "fewest" in this sentence is the correct option since we use the word "the" with superlative adjectives.