We were taught the relative clauses with "jenž ...", so now their usage should be accepted. ;-)
They should, but be aware that almost no-one uses "jenž" and derivatives in speech. I don't use it even in writing. It is just too formal or bookish or old... So, it may be forgotten as a possibility here and there.
Moreover, this is quite far in the course and there will be more missing translations than in the first lessons. This particular sentence is missing many possibilities.
Why I cannot use "on" here? "On si vybral"
Second position rule for si. Most likely, that is. You should report the complete sentence.