I did a brief analysis.

We must to have in mind that the correct question must always be true.

Concerns for the environment have led chemists to develop plastics that are degradable. All degradable plastics, which are potentially useful packaging materials, need just the right conditions to break down. Some need exposure to sunlight, some need to be buried in soil, and some need to be submerged in water. It should be cautioned that some degradable plastics leave residues of unknown toxicity. If all of the statements above are true, which one of the following must also be true?

(A) Some materials that are potentially useful for packaging leave residues of unknown toxicity.
It is always true, some means 1 or 100. The stimulus states: "It should be cautioned that some degradable plastics leave residues of unknown toxicity", and all degradable plastics are potentially useful packaging materials, thus it must be always true.

(B) Some degradable plastics need both sunlight and submersion in order to break down.
It is not true. "Some need exposure to sunlight, some need to be buried in soil, and some need to be submerged in water"

(C) Some materials that need sunlight in order to break down are not potentially useful packaging materials.
It not true, because some are potentially useful packaging materials "all degradable plastics are potentially useful packaging materials"

(D) Some materials that leave residues of unknown toxicity are not degradable plastics.
It could be true, but it also cannot be true

(E) Some materials that need to be buried in soil to break down leave residues of unknown toxicity.
It is pretty good contender, but I think it is not always true, because some material that need to be buried in soil could not always be degradable plastics, so it cannot be true with the information given.

Therefore, the correct answer should be A.

A for me too.

I was between A and E.
E is too broad

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