This question can be easily answered by using the process of elimination. It can be inferred that 'Them' is mostly used to objectify something, so it is an objective pronoun. 'That' is most usually an adverb, but can also be an adjective and even conjunction. 'Those' is considered a demonstrative adjective because it is usually used when referring to something in a sentence. This obviously leaves you with 'Through', this can be easily remembered as a preposition because of its prepositional definition; "in and out of".
The author uses the word "metamorphosis" to describe the "four-stage process of change." They are introducing new vocabulary terms in a friendly context. They also may have included it because it's the most accurate scientific term for the process of growth undergone by the beetle.
