What part of speech answer the question to what extent?

The part of speech that answers the question "to what extent" is usually an adverb.

Adverbs modify verbs, adjectives, or other adverbs, providing information about how, when, where, or to what extent something happens.

Here are some examples:

* "How much" - This phrase acts as an adverb, indicating the extent of something. For example, "How much did you eat?"

* "Very" - This adverb describes the degree or extent of something. For example, "He was very tired."

* "Partially" - This adverb indicates a limited extent. For example, "The window was partially open."

* "Completely" - This adverb indicates a full extent. For example, "The room was completely empty."

While adjectives can also describe the extent of something, they usually modify nouns, not verbs or other adverbs. For instance, "He had a large appetite" describes the size of his appetite, but doesn't answer the question "to what extent."