A "stimulus" is basically an event that happens in the outside environment that triggers a response. So, when you touched the dog's ear, that's a stimulus. The twitch is a response! So, A is correct. But just to be sure, let's look at the rest.
Artificial selection (B) is basically breeding: when humans breed organisms in order to obtain desired traits, like breeding dog species--this is not what our example is talking about.
Natural selection (C) is a similar process. When an organism mutates, but it actually helps it survive, it is more likely than the others to survive long enough to have babies. These babies are likely to have the helpful mutation, and so are their babies, and theirs, until a good portion of them have the mutation. This is NOT the example above!
And D, reproduction--well, let's just say it would involve a female dog and lots of babies, but that's not related to our example, so that isn't right either. The answer is A: a response to a stimulus.
Answer: A
