Step 1: Understanding the Idiom
The idiom "a dark horse" refers to a candidate or competitor about whom little is known but who unexpectedly wins or succeeds. The term originates from horse racing, where a "dark" (i.e., unknown) horse unexpectedly won a race.
Step 2: Evaluating the Options
(A) Black colour horse: This is the literal meaning of the words and not the idiomatic meaning.
(B) An unknown entry: This perfectly captures the essence of the idiom. A dark horse is an entrant in a competition (like an election, a race, an award) who is not well-known and is not expected to win, but then does.
(C) Well Known face: This is the opposite of a dark horse.
(D) Known depredator: A depredator is a predator. This is completely unrelated to the meaning of the idiom.
Step 3: Final Answer
The idiom "a dark horse" refers to a surprise winner or an unknown but successful competitor. "An unknown entry" is the best description of this. Therefore, option (B) is the correct answer.