megakee9
Also, A,B are wrong because 'Washington's years' cannot be regarded as 'a Black leader.' Wrong modifier.
Hello
megakee9,
Your analysis tells me that you think that in Choice A,
regarded to be the most influential Black leader of his time refers to
The last years.
I am afraid that is not the case. In Choice A, the noun modifier
regarded to be the most influential Black leader of his time modifies the immediate preceding noun
Booker T. Washington.
Following is another correct official sentence in which a similar noun modifier modifies the immediate preceding noun:
The exploits of Nellie Bly, a pioneer journalist, included circling the globe faster than Jules Verne's fictional Phileas Fogg. In the above-mentioned correct sentence, the noun modifier
a pioneer journalist correctly modifies the immediate preceding noun
Nellie Bly and not
The exploits because that modification is not logical.
Same is the case with the official sentence in question. It does not make sense for the noun modifier
regarded to be... to modify
The last years.
Hope this helps.

Thanks.
Shraddha