Answer:

Explanation:
The given statement is a sentence that states a fact or a reason, and it requires an adverb to modify the verb "leave." In this case, the adverb "on Thursday evening" specifies the time or manner of the action.

Adverbs:
Adverbs are words that modify verbs, adjectives, or other adverbs. They provide information about how, when, where, or to what extent an action is performed. In this case, the adverb "on Thursday evening" modifies the verb "leave" by specifying the time at which the action will take place.

Example:
If the sentence were to be modified to use an adjective instead of an adverb, it would be incorrect. For example, if we were to say "I will leave on Thursday evening," it would imply that Thursday evening is more expensive, which does not make sense in this context.

Other Options:
- Adjective: Adjectives describe or modify nouns or pronouns. They provide information about the qualities or characteristics of a person, place, thing, or idea. In this case, "more expensive" is not functioning as an adjective because it is not describing or modifying a noun or pronoun.
- Preposition: Prepositions show relationships between a noun (or pronoun) and other words in a sentence. They indicate location, direction, time, manner, or cause. In this case, "to" is a preposition that shows the relationship between "it is more expensive" and "travel on Friday," but it is not the correct answer because it does not modify the verb "leave."
- Conjunction: Conjunctions are words that connect words, phrases, or clauses. They are used to coordinate, subordinate, or join elements in a sentence. In this case, there is no conjunction present in the given statement.

Therefore, the correct answer is option 'A' - adverb. The adverb "on Thursday evening" modifies the verb "leave" by specifying the time at which the action will take place.