(A) High school reunions are usually looked forward to by
alumni that have worked hard since graduation and now consider themselves successful.
-- that is used for singular subjects, here alumni is plural. Passive voice in the first half of the sentence is not ideal.
(B) High school reunions are usually
looked forward to by alumni who have worked hard since graduation and now consider themselves successful.
-- Passive voice in the first half of the sentence is not ideal. Better sentence would be to say " Alumni look forward to ***"
(C) Alumni who have worked hard since graduation and now consider themselves successful usually look forward to high school reunions.
-- Correct
(D)
Alumnus that
have worked hard since graduation and now consider
themselves successful usually look forward to high school reunions.
-- Should use 'Alumni' plural form instead of 'alumnus', as remaining portion of the sentence indicates plural subject.
(E) High school reunions are usually looked forward to by
alumni that have worked hard since graduation and now consider yourselves successful.[/quote]
-- that is used for singular subjects, here alumni is plural.