My answer is (C). It took me 01:11.
Here are a few sentences to illustrate the use of "forbid".
(1) He forbade me to tell anyone about his plan.
(2) She is forbidden from leaving Canada.
(3) He was forbidden to leave the house, as a punishment.
(4) Women are forbidden from going out without a veil.
(More discussions at
https://forum.wordreference.com/threads ... g.1260635/)
So "forbid somebody to do something" is more common. Let's review more "prohibition words" and realize that "forbid" is a special one.
• Prevent somebody
from doing something
• Prohibit somebody
from doing something
• Keep somebody
from doing something
• Preclude somebody
from doing something
• Deter somebody
from doing something
• Debar somebody
from doing something.
• Dissuade somebody
from doing something
• Forbid somebody
to do something
(A) The only thing against this sentence is "from doing" rather than "to do".
(B) "the" should not be used because the Oath applies to all physicians not just a particular group.
(C) Compare with (A). "Forbid somebody to do something" is used.
(D) Compare with (A). Essentially the same, a fact that usually disqualify both options.
It is also wordy, an argument that only helps eliminate this version.
(E) "forbid that" is not a well-established usage pattern.