the one / one

[joham]

My most famous relative of all, ________ who left his mark on America, was Reb Sussel, my great grandfather.
A. one B. the one C. he D. someone
The given answer is B. I'd like to know if we can also use A, which means 'a person'.
Could I ask native speakers to help me please? Thank you in advance.

[David L.]

The first clause says, 'My most famous relative of all'.
Only one person of 'all my relatives' can be 'the most famous'.

It was that one, 'the most famous one', he and he alone amongst all my relatives, who left a mark on America.

Otherwise, if we use 'one' , whom does this 'one' refer to?
There become two possibilities:

1. My most famous relative of all, but just one of all my relatives who left their mark on America, was ...

2. My most famous relative of all, one of all the people (in history) who have left their mark on America, was ...


Can you see that by using 'the one', it specifically refers back to (a) a relative, and (b) the relative regarded as 'most famous', and so keeps the meaning highly specific, and so clear.