Select the option that will improve the highlighted part of the given sentence.
The moment James saw Regena, he fell head on to his heels in love with her.

The correct substitution is "head over heels".
Explanation:
· The phrase "head over heels" is an idiom that means "completely in love" or "deeply involved".
· The given phrase "head on to his heels" is incorrect and not a recognized expression.
Grammatical rule used:
· Idiomatic expressions should be used correctly in the exact form.
· The phrase "head over heels" means being extremely enthusiastic or deeply in love.
Example:
· John fell head over heels in love with Lisa.
Incorrect options:
· Head and heels (incorrect phrase).
· Heels of the head (nonsensical phrase).
· Heads in the heels (incorrect phrase).