The correct option is
C Number of spidersIn the given line, the word omitted should come after the word 'number'.
So, options A) and B) are incorrect. The part of the sentence 'inhabit our environment' is correct.
Between the options C) and D), 'number of spiders' is correct while 'number in spiders' is incorrect. When we talk about number, we use the preposition 'of' and not 'in'. So, the correct answer is C) number of spiders.