The blank requires a preposition or an adjective to form a phrasal verb along with the verb 'break' and match the context of the sentence. 'Break through' (make or force a way through a barrier), 'break out' (start suddenly) and 'break even' (reach a point in a business venture when the profits are equal to the costs) are contextually irrelevant phrasal verbs and so the words associated with these phrasal verbs are negated. Option (c), 'break down' (undergo chemical decomposition) is the most appropriate phrasal verb that denotes the work done to plastics and so, can fill the blank.
Hence, the correct answer is option (c).
Note:
Phrasal verb: A phrase that consists of a verb with a preposition or adverb or both, the meaning of which is different from the meaning of its separate parts. Example: She has always looked down on me. Here, the phrasal verb 'to look down on someone' doesn't mean that you are looking down from a higher place at someone who is below you; it means that you think that you are better than someone.
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