Hi PaterD,
Thank you for your question. When looking over each possible answer quickly, it's clear there are a couple glaring differences between each answer:
1. starting with was/were
2. ending with for/for asking/which asked
Let's start with was/were. This word is referring back to a singular "experiment," so we should only keep answers that use the singular "was." Therefore, we can rule out answers B & C because they use the plural "were," which is wrong.
Now that we have A, D, & E left, let's tackle the ending of each:
A: for asking
This one is wrong because it doesn't follow parallel format for verb tenses. The experiment was conducted in the past, so it needs to have a past tense verb to match.
D: for
Whenever we use a comma + for, we're saying that "for" is being used as a conjunction (it means the same thing as "because"). Since we cannot replace "for" with "because" and have it still make sense, this one is incorrect.
E: which asked
This one is correct because saying "which asked" tells readers the phrase is just giving us more information about what the experiment required. You could easily take this phrase out of the sentence and it wouldn't change the overall meaning, and it is written to clearly show it's just adding non-essential information. Therefore, it's correct.