Answer:
Two hundred and fifty million years ago the landmasses of Earth were clustered into one supercontinent dubbed Pangea.
Explanation:
Hoped I helped - GOOD LUCK
The main difference between an adverb and a preposition is that a preposition must be followed by a noun (The cat jumped <u>onto</u> the table), and an adverb doesn't - an adverb also modifies a verb (She sang <u>happily</u>), whereas a preposition has to do with nouns.
Having this explanation in mind, the correct answer is adverb.