The problem with (A) is not the word "exactly." Instead, the problem is that (A) is not a required assumption. Although we need to know that molting happens in some type of consistent and measurable way, there is nothing in the passage that specifies how this way must look. In other words, there is nothing supporting the idea that rattlesnakes have to molt exactly once a year as opposed to exactly once a month for example. This is why (A) is ultimately incorrect.
Does that make sense? Please let us know if you have any other questions!