Despite the trust bestowed on the minister he turned out to be a snake in the grass during the revolution.

The idiom "a snake in the grass" refers to someone who appears to be friendly or harmless but is actually a hidden enemy or traitor.

Let's look at the options:
Option A: a secret enemy - This is a good description of someone who is a "snake in the grass." They are not openly hostile, but secretly work against you.
Option B: a treacherous person - This is also a very accurate description. "Treacherous" means someone who betrays trust, which is exactly what a snake in the grass does.
Option C: an unforeseen danger - While a snake in the grass *can* present an unforeseen danger, the idiom specifically refers to the person, not just the danger itself.
Option D: an unexpected misfortune - This is a general negative event and doesn't capture the element of betrayal inherent in the idiom.

Between A and B, both are good. However, option B, "a treacherous person," is slightly better because it emphasizes the betrayal aspect which is central to the "snake in the grass" meaning.
So, Option B is the best answer because it accurately captures the meaning of the idiom as a person who betrays trust after seeming friendly. Option A is also close, but 'treacherous' is a stronger fit.