This one is largely meaning based. So to get it right one has to look at the meaning created by the use of each answer choice.

I have seen people argue that some of the answer choices contain non parallel constructions created using the word which, but I am not so sure that those constructions don't work. So in choosing a best answer, I am going to stick with identifying what is 100% clearly wrong.

As a result of a supernova explosion, every human being on Earth was bombarded on February 23, 1987, by about 100 billion neutrinos; fortunately, neutrinos are harmless elementary particles that are produced in nuclear reactions and that interact very weakly with matter.

(A) neutrinos are harmless elementary particles that are produced in nuclear reactions and that

The sentence created using this one makes sense. It conveys that every human being was bombarded, but fortunately neutrinos are harmless...particles...that interact very weakly with matter.

(B) neutrinos, which are harmless, are elementary particles produced in nuclear reactions and which

This sentence created using this one does not make as much sense, as it conveys that every human being was bombarded, but fortunately neutrinos...are elementary particles produced in nuclear reactions. How is that fortunate?

(C) neutrinos are harmless elementary particles produced in nuclear reactions and which

The sentence created using this one does not quite make sense. It does not clearly convey that what is fortunate is that the particles are harmless.

One could also argue that produced in nuclear reactions is not parallel to which interact very weakly, but in a sense they modify particles in similar ways. So I prefer not to use this potential issue as my decision point for eliminating this answer choice, especially as there is a much clearer decision point available.

Generally when doing SC questions it's a good practice to find and use clear decision points rather than "Hmm, I think that's probably wrong." decision points. In other words, if you are not sure of a decision point, you are best off ignoring that one if you can find one that is more definite for you.

(D) these harmless elementary particles are produced in nuclear reactions, and neutrinos

Using this one we get the meaning fortunately these...particles...are produced in nuclear reactions. It's not clear to what these particles refers, and even if we guess that these particles are neutrinos, it does not make sense that it is fortunate that they are produced in nuclear reactions.

Then, finally the word neutrinos shows up after and, but at that point it is redundant.

(E) these elementary particles, harmless products of nuclear reactions, are neutrinos that

Once again it's not entirely clear that these elementary particles refers to neutrinos, at least at first. Then of course we get the nonsense remainder of the sentence that conveys that fortunately these...particles...are neutrinos that interact very weakly with matter.

So let me get this straight. Fortunately neutrinos are neutrinos that interact very weakly with matter, or possibly the sentence is meant to convey that the neutrinos from the supernova explosion are neutrinos that interact very weakly with matter. Nah. Neither one of those really makes sense.

So the only one that really makes sense is A.