Professional baseball contracts have a standard provision, the reserve clause,
under which the rights to a player are permanently retained by the team that originally hires him or acquires his contract.
A) under which -
CorrectB) which assures - Incorrect
C) that assures - Incorrect
D) for which - Incorrect
E) that - Incorrect
For Better Understanding let's put each of them in the original sentence.
A) Professional baseball contracts have a standard provision, the reserve clause,
under which the rights to a player are permanently retained by the team that originally hires him or acquires his contract.
First of all, "under Which" is grammatically correct. In this case, "which" is not used for the reserve clause but rather for a standard provision.
Example: I abhor the conditions under which we commercially raise farm animals today.
B) Professional baseball contracts have a standard provision, the reserve clause,
which assures the rights to a player are permanently retained by the team that originally hires him or acquires his contract.
There is two verbs i.e., assuers and are, So It's clearly violating the Subject-verb agreement. C) Professional baseball contracts have a standard provision, the reserve clause,
that assures the rights to a player are permanently retained by the team that originally hires him or acquires his contract.
Same as BD) Professional baseball contracts have a standard provision, the reserve clause,
for which the rights to a player are permanently retained by the team that originally hires him or acquires his contract.
"For" is creating meaning error.E) Professional baseball contracts have a standard provision, the reserve clause,
That the rights to a player are permanently retained by the team that originally hires him or acquires his contract.
Interesting Option, but Option A is more precise.
Correct Answer : A