The SI unit of area is

The SI (International System of Units) assigns a base unit to each fundamental physical quantity and builds units for derived quantities by combining base units according to the quantity's defining relation; the SI base unit of length is the metre (m), and area is a derived quantity formed from length.

1. Area of a plane figure equals length multiplied by breadth, a product of two length measurements.

2. Substituting the SI base unit of length (metre) for each length term gives the derived SI unit of area as metre × metre, i.e. square metre (m2).

3. Comparing this derived unit with the options given, the square metre (m2) is the one that matches.

This also follows from dimensional analysis: the dimensional formula of area is [L2], and substituting the SI unit of length (m) reproduces m2, confirming the result. The remaining options — mm2, cm2, and km2 — are valid area units related to m2 by fixed powers of ten (1 m2 = 106 mm2 = 104 cm2 = 10-6 km2), but each carries a metric prefix rather than being the unprefixed SI derived unit.