To prove by contradiction that among the natural numbers $a$, $b$, and $c$, exactly one is even, we need to assume the opposite of what we want to prove, and show that this leads to a contradiction.

Let's go through each of the provided options:
- Option A: Assuming $a$, $b$, and $c$ are all odd means there are no even numbers, which contradicts the statement that exactly one is even. This is a valid assumption for proof by contradiction.
- Option B: Assuming $a$, $b$, and $c$ are all even contradicts the premise outright since it provides more than one even number, but it is not a comprehensive contrary since it does not cover the cases where two numbers are even.
- Option C: Assuming $a$, $b$, and $c$ are all odd or at least two are even covers all possibilities that contradict the original statement. If all numbers are odd, there are no even numbers; if at least two numbers are even, then there are more than one even numbers. This option fully captures the essence of the contradiction.
- Option D: Assuming at least two numbers are even is only a partial contradiction, as it does not address the possibility that all might be odd.

To prove by contradiction, we must consider all cases contrary to the proposition that exactly one number is even. Option C is the most appropriate choice as it covers the full range of contradictory scenarios.

Thus, we have:
$$\boxed{\text{C: }a,\ b,\ c\ \text{are all odd or at least two are even}}$$