Right answer: fácilmente
Firtsly, is important to explain that an adverb is a part of the sentence that complements a verb, an adjective, other adverbs and even sentences.
Adverbs express circumstances, such as mode, place, time, quantity, affirmation, doubt; answering questions such as: when?, where?, how?, in what way?, among others.
In other words:
The main function of adverbs is to act as modifiers or complements of a verb or verbal phrase
Now, in Spanish there are several types of adverbs, but in this specific case we are talking about the adverb of manner.
The adverb of manner can be formed by placing the suffix -mente at the end of some adjectives.
As it is the case of the adjective fácil (easy) that changes to an adverb of manner as fácilmente (easily).