iongmat wrote:In the mid-1920's the Hawthorne Works of the Western Electric Company was the scene of an intensive series of experiments investigating the effects that changes in working conditions would have on workers' performance.

It's ok to write that the experiments investigated these effects.

In some situations--such as this one--you have "compromises" in terms of the exact literal quality of a modifier. If...
... there's no way to write a single sentence that's exactly literal,
or
... the trade-off is too severe (in terms of extra awkwardness/wordiness/mentioning of unimportant concepts),
then you'll sometimes see sentences that are not 100% literal.

Here, for a 100% literally correct phrasing, you'd have to mention the experimenters ("...experiments in which the experimenters investigated xxxx"). That's not only wordy, but also distracting (we don't care about the experimenters; we care only about the experiments). So, we write "experiments investigating xxxx".

Same thing with, say, "a new style of airplane that can fly without being detected by radar". Sure, it's actually the plane that flies, not the "style"--but that's going to be impossible to do in a single sentence.

Fortunately, you'll never have to make this decision, because there are 5 choices. You just have to pick the choice in which the modifier is most accurate.

Here, of the things actually mentioned in the text of the choices, "the experiments" is clearly the most accurate referent for "investigating xxxx". So there is no issue.