Answer:
The return on my investment was greater than i expected.
Explanation:
The superlatives and comparatives are modifiers to nouns to make them compare to something else, or to a group of things, in this case the word great is a two syllable comparative, and regular, so we just add -er at the end if its a comparative or -est at the end if it is a superlative, since in the sentence the author wants to compare the return that he actually got versus the one that he thought he was getting, we have to make it a comparative, by addin -er, so the correct use of the word would be greater.
