I get that the idea is that some questions create ambiguity using wordplay or subjectivity, but do you really think this is one of them? Your examples seem like a stretch even in the context of being unfair on purpose.
Yes, because it is well known that these tests were in fact designed to be unfair on purpose (to a specific racial group). So it's not a stretch to think that these "unfair on purpose" examples are realistic.
There are three reasonable interpretations I see. The instruction is clearly to draw a line under something. That something may be whatever is followed by "under", so you underline "the last word in this line". Or "in this line" just narrows it down, so only "the last word" is to be underlined. Or the whole "last word in this line" is meant as an instruction to be interpreted, so it's "line". In that case, be careful not to underline the period, as sentence marks clearly aren't part of a word. Or maybe they are.
Oh wait, it could also refer to "the last 'word' in this line", so you would need to underline "word".