Fill in the blanks with suitable words given in the options. I have read ______ books on modern philosophy and attended numerous lectures, which have significantly ______ my understanding of contemporary theories and their applications.

This blank requires an adjective to qualify the noun 'books'. The adjective 'several' means a number of things, which is appropriate for describing books. The adjectives 'energetic' and 'gradual' (taking place, changing, moving, etc., by small degrees or little by little) do not fit the context. The words 'specification' (a particular item, aspect, calculation, etc., in such a description) and 'stipulation' (a condition, demand, or promise in an agreement or contract) are nouns, not adjectives. The present perfect tense (indicated by the helping verb 'have') requires the past participle form of the verb. Among the given options, only 'expanded' is in the past participle form. 'Develop', 'inflate' (to cause to expand or distend with air or gas) and 'enlarge' are the base forms. 'Grew' is the past tense form.