Concept:
Carbohydrates are polyhydroxy aldehydes or ketones.
Step 1: Minimum carbon requirement.
To have both a carbonyl group (aldehyde/ketone) and multiple \(-OH\) groups, at least 3 carbons are needed.
Step 2: Smallest carbohydrate.
Glyceraldehyde (CHO–CHOH–CH\(_2\)OH) contains 3 carbons — the simplest carbohydrate (a triose).