. . . . . clever you are, you can’t solve this puzzle.

The correct answer is (B) However.

Let's break down why:

* The Sentence's Meaning: The sentence means that *no matter* how intelligent someone is, they still won't be able to solve the puzzle.

* Why "However" Works: "However" introduces a contrasting idea. It implies that despite being clever, solving the puzzle is still unlikely.
Example: "However clever you are, you can't solve this puzzle" means, "No matter how smart you may be, you still won't be able to solve this."

* Why the other options Don't Fit:
* (A) Very: "Very" describes a degree. It doesn't connect two contrasting ideas effectively.
Example: "Very clever you are" doesn't create a proper sentence.
* (C) Even if: "Even if" introduces a condition. While it hints at something similar to "however", in this specific context, it is not the grammatically best option.
Example: "Even if clever you are" doesn't read like the typical use with "even if". "Even if you are clever" would be an improvement, but "However" fits the sentence better.
* (D) Since: "Since" indicates a reason or cause. It doesn't express a contrast between intelligence and the ability to solve the puzzle.
Example: "Since clever you are, you can't solve this puzzle" doesn't make logical sense.

In summary: "However" is the best choice because it smoothly introduces the contrasting idea that solving the puzzle is difficult, *regardless* of one's intelligence.