Question pack 1 = The industrial pollutants known as PCB’s

OA: A

With Intuition I was able to conclude that Option A, but can experts help me with grammatical dissection?

Option A: The events are from the past so we must use past tense. Also, two set of events happened in the past one after the other. First event must use past perfect (had) and second event must use simple past.

Option B : Present Perfect (have) suggests that an action or a result of action is still ongoing.

Option C, D and E: use of passive voice / nonsensical.

let me give it a shot.

let us look at option A and the underlined clause -

"when studies showed that the compounds had entered the food chain and were harmful to some animals."

We need the "that" here to show an idea - the idea that compounds had entered the food chain.

Also, while reporting, we tend to shift the tenses back.

i.e. -
studies in 1970 showed, "the compounds entered the food chain and are harmful to some animals". becomes

studies in 1970 showed that the compounds had entered the food chain and were harmful to some animals.

Option B -
"studies showed the compounds ..." - does not make logical sense.

Note that this construction - "to have entered" is not the present perfect. Rather it is something called the perfect infinitive.
It implies that the verb "entered" happened before the preceding verb "showed".

Option C -
"studies showed the entry of the compounds ..." - studies did not show the entry itself. does not make logical sense.

Also, this option suggests that the entry itself is harmful to some animals. Illogical.
"the entry of the compounds into the food chain as harmful to some animals".

Option D -
makes the same error as options B and C.
"its" refers to "the entry of compounds"; we are not referring to the harmfulness of the entry itself.

Option E -
same error as above.
"studies showed the compounds ..." does not make sense.studies did not show the compounds themselves but the fact that those compounds entered the food chain.