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I'm usually good about "if assumed" types but this one has me wondering if I ever understood this question type at all.

The right answer is C "no one but herbert and samantha were in the office on the day of the murder".

I narrowed it down to C/D but couldn't decide which one - went with D - wrong one.

The reason D looked like a possibility was because I thought if the fingerprints were Jansen's then the conclusion that Samantha is the killer would not be true. D seemed to make the conclusion true.

What am I doing wrong on this one? Did anyone else have trouble on this?
Thanks!

I think the key is, the argument gives us that:

Premises:
These 2 particular dudes were in the office on the day of the murder, Y & Z.
Only the people in the office that day could have killed him.
If A happened, then it was Y who killed him.
If A didn't happen, then Y didn't kill him.

Conclusion:
A didn't happen, so Z killed him.

Well this argument only works if you assume that only Y & Z were the only people who could've killed him; So to make the argument work, we have to assume that Y & Z were the only people in the office that day, and thus were the only people who could've killed him.

Hope this helps, doing it completely from memory so apologies for any inaccuracies.

I'm still having a hard time understanding this one...

Isn't the premise saying:
If Z was the murderer, then B (not leaving fingerprints) happened?

I can just see C and E both working here...

You need to show they were the only two people in the office (C) and that the fingerprints did not belong to Samantha (E) since the premise says she would not have left fingerprints if she were the murderer.