One must note that Zero-day attack exploits unpatched software vulnerability before it's known to guards. (Option B)
Consider a computer system as if it were a castle manned by security personnel.
A "zero-day" assault, therefore, is akin to discovering a hidden passageway (vulnerability) that the guards are unaware of.
Diffie-Hellman is only an elegant means of communicating in secret; but, if it is breached, it might be equated to the adversary intercepting messages.
Using brute force is similar to trying every key until one fits. An act of man-in-the-middle occurs when a cunning spy eavesdrops on communications between two parties.
Therefore, taking advantage of an unpatched software vulnerability is equivalent to slipping past security measures without the guards even realizing it.