Unfortunately, I can't see the picture of you pointing to muscles in your arm, but I can give some information as to the different muscles the arm is comprised of. The most prominent muscles in the arm that can be seen easily on the surface include the following:
Bicep
Tricep
Brachioradialis
Deltoid
Extensor carpi radialis longus
So what muscle is it?
Without being able to see a picture of the muscle in question, it's difficult to say what muscle you're referring to. I would suggest that it's likely to be one of the above, as they are the easiest muscles to view on the surface of the arm.
Here's a pretty cool video that shows you a 3D anatomy model of the arm: