which is a combining form not a prefix or suffix

A patho
b centesis
c opsy
d retro

3 answers

What do you think? Someone will be glad to check your answer.

http://www.enchantedlearning.com/grammar/prefixsuffix/
If you are unsure what these words/choices mean, please look them up:

http://www.merriam-webster.com/
Patho.