I'm having trouble with this one.
My answer would be that the overall charge on serotonin would be negative since acidity increases at low pH levels?
Any advice?

Thanks!

Serotonin has one -OH group on a phenyl ring. The hydrogen on that is acidic. It also has an -NH2 group that can accept a proton (hydrogen atom) and a -NH group where the N is in a 5-member ring. You can look up the structure to see what I'm talking about.

As acidity increases, the -OH will be less and less ionized so it will be neutral. The -NH2 group will become -NH3+ and the -NH group will become NH2+. The latter group will disperse the electrical charge around the conjugated ring structure so it will actually accept a proton more readily than the -NH2 group.

Overall, the structure will become more positive. There will, of course, be other anions hanging around.