The correct option is C Several
'Much' is an indefinite numerical adjective which is used for uncountable nouns, hence it is an incorrect answer because books are a countable noun. 'Enough' is a positive adjective implying that the numerical strength of the noun has been reached the required amount or fulfills a requirement of some form, hence it is an incorrect option. 'Several' refers to a countable noun, in this case being the books, this is the correct answer. The adjective 'lot' is incomplete without 'of'. If 'lot of' was used, it would be correct, otherwise it's incorrect.