The question asks for the opposite (le contraire) of grand (big, tall, great). In French, the standard antonym of grand is petit (small, short, little). This is one of the most fundamental adjective pairs in French vocabulary.
The other options are not antonyms of grand:
mauvais means "bad" - the opposite of bon (good).
joli means "pretty" - the opposite would be laid (ugly).
long means "long" - the opposite is court (short in length).
Note the distinction between petit (opposite of grand in size or height) and court (opposite of long in length or duration). These are not interchangeable.
