A simple visual way to determine if a particular graph is a function is by imagining vertical lines passing through it. If the vertical line touches more than once, it is not a function.
r - A line passing through the vertical edge of the r touches many times.
o - A line passing through the middle touches twice
p - Both the vertical line on the left side, and the circular part would touch more than once
v - Any vertical line passing through it would touch exactly once. That makes it a function.