Atheism is not a religious belief; it only refers to the lack of a specific belief (that one or more gods exist). I am an atheist because I lack a belief in gods, and I am nonreligious because I lack any religious beliefs. Anyone who is nonreligious must also be an atheist for the reasons I described earlier, but not all atheists are nonreligious (Buddhists do not believe in gods and are technically atheists).
Agnosticism has to do with knowledge, not belief. I am an atheist because I lack the belief that god(s) exist. I am also an agnostic because I lack the knowledge of whether or not god(s) exist (i.e. I cannot prove nor disprove the existence of a god). Agnosticism says nothing about one's religious beliefs and probably should not have been included on the list of religious beliefs in the survey. There are agnostic Muslims, agnostic atheists, agnostic Christians, etc.