Is it possible for a carbon atom with a negative formal charge to have ONE bond and ONE lone pair? I was completing some problems from my textbook and found that the answer of this problem seemed incorrect to me. 

From my limited understanding, carbon atoms with a negative formal charge must have 5 electrons (3 bonds and 1 lone pair), but this answer didn't match up with this rule.

I would really appreciate it if someone could tell me what I'm missing here.
The hydrogen atoms are not shown, so this is really $\ce{CH3CH2CH2CH2-}$ where the rightmost carbon atom has a lone pair and a formal negative charge.

The species is the n-butyl anion: