Answer: B
Explanation:
infinitesimal means extremely small so
B. gigantic means very great size or extent; huge or enormous which is the best answer occured to me.
C. tiny is not correct since they're synonym
D. interesting is also incorrect because it's not related to the word
A. infinite is limitless or endless in space, extent, or size; impossible to measure or calculate. It can be interpreted as the antonym of the word infinitesimal but this answer seems not as good as option b.
Hope I got it right
