The functional isomer of methylmethanoate is propanoic acid.

Explanation

To determine the functional isomer of methyl methanoate, we need to analyze both compounds and their structures.

### Step 1: Identify the molecular formula of methyl methanoate.
Methyl methanoate is an ester with the following structure:
- Methyl methanoate (also known as methyl formate) has the structure:
\[ HCOOCH_3 \]
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