Tough one. I think B is correct answer.
A. is out for using "experiments that would
investigate...". Here investigation is happening at the same time, so "investigating" should be a better option. If we choose "would" (which is past of "will"), it sounds as if the investigations will happen at a later stage.
B. Correct. B is using "would" in the phrase "changes in working condition would have". Clearly the changes (i.e. the result of experiment) will come at a later stage, and not at the same time.
Present participle form of "investigating" is correctly used, modifying the experiments.
C.Sounds awkward and wordy. "what" is redundant. Better option would have been "investigating the effects" rather then "investigating what the effects"
D. "that investigated" sounds as if the experiments just invetigated once. "working conditions effects' on" sounds awkward. Also it sounds as if changes have happened on "working conditions effects' " and not on "working conditionS"
E. "what" is redundant. Infinitive form "to investigate" would have been better if we wanted to express the intention for conducting experiments. But here experiments are already in progress.
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