The first member of the ester homologous series is
Methylmethanoate
Esters are the compounds having −O||C−C group.
Now, if we look at the first member of ester homologous series, then we would probably take H instead of R in R-COO-R
H−O||C−O−H χ
Can you see that it becomes a carboxylic acid?
H−O||C−O−CH3 ✓
So, let's now replace one H with CH3
This is the structure of methyl methanoate.
So, option (c) is correct.