Answer:
adjective.
Step-by-step explanation:
An adverb is one of the parts of speech which is used to modify a verb, adjective, and another adverb. The purpose of an adverb is to tell the reader about the hows, where, and whens of an action or its quality.
An adverb is easily identifiable as most of it usually ends with -ly such as kindly, silly, fortunately, etc. Some words plays double role of both being an adverb and adjective. Such as fast, late, most, etc.
So, from the given options the correct answer is that the word an adverb modifies is adjective.
An example of adverb modifying adjective-- 'This book is very interesting.'
Thus option second is correct.