1.why is the correct answer “when”, not “until”
because "I had lived there for over thirty years" denotes a time level prior to that of "I wanted to return to my motherland again"
if you used "until" this would mean one continuing level of time and then one of the tenses would be wrong.
D12. I had lived there for over thirty years _________ I wanted to return to my motherland again.
2.why is the correct answer “but”, not “however”
My name is Robert, ________most of my friends call me Bob for short.
I'd say it's above all, a matter of punctuation
you would need another comma after however, which, as far as I can see, simply isn't there in the text.
possibly even a semicolon instead of a colon after Robert
2.why is the correct answer “that”, not “who”
She is not the woman _________ she was before.
A missing comma tells you that you are looking at a defining relative clause.Note that if you leave the identifying relative clauses out, the sentence would be incomplete.
'that' can only be used in defining relative clauses and those are clauses where you need the information to understand what you're talking about.
hope this helps.