My analysis is below:
Lindane,
a chemical used for both agricultural and pharmaceutical purposes,
effectively treats maladies,
and it
has long appeared [u]that it was safe for use on children,
but we now know that
this is incorrect.
First, use of Present perfect is required to describe an action that started in the past and continues today. There are several mistakes here:
"has long appeared that it was safe for is wordy. "Has appeared" should be followed by an adjective (safe) - straightforward
Use of "for use on children' is unidiomatic. "Use to" is best to describe the action on children.
Also I think that the use of "this" in the non-underlined part is not correct. Demonstrative pronoun "this" should be followed by a noun. Alone can't be use to refer to a previous stated noun or clause.
According to 800score, the correct choice is D. However I think it is not correct to use Past perfect since there is no sequence of events.
My correct answer is the following:
Lindane, a chemical used for both agricultural and pharmaceutical purposes, effectively treats maladies such as head lice and scabies, and it
has long appeared safe to use on children, but we now know that this
belief/fact is incorrect