In any question like this, the trick is to simplify, by separating the subject part of the sentence from the verb part.
Let's look at this example:
Every workday at dawn, the patriarch of one of the city’s five richest families leaves his mansion and walks to city
A) richest families leaves his mansion and walks
B) richest families leave his mansion and walk
C) richest families leaves his mansion and walk
D) richer families leave his mansion and walks
E) richer families leaves his mansion and walks.
The subject (the thing or person taking the action) is 'the patriarch' (and NOT 'families' – that word is placed there to confuse you
). The verbs are 'to leave' and 'to walk'.
Here they are again, separated:
Every workday at dawn, the patriarch ...
A) ... leaves his mansion and walks
B) ... leave his mansion and walk
C) ... leaves his mansion and / walk
D) ... leave his mansion and / walks
E) ... leaves his mansion and / walks.
In both A and E, the singular subject 'the patriarch' agrees with both verbs 'leaves' and 'walks'.
Having deduced that, we've then just got to choose the correct adjective: is it 'one of the ... richest families' or 'one of the richer families'?
Well, richer is a comparative adjective, but no comparison is offered here. In other words, for 'richer' to be acceptable, the sentence would need to say whom or what his family is richer than. As the sentence doesn't give us that information, 'richest' is the only adjective that works. Hence answer A is the only one that will work.
So whenever you have to answer a question like this, make sure you break the sentence down into its component parts. It may feel like a waste of time, but it will actually save you time overall. It will also reduce your chance of falling into the traps that are laid for you.
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