What is the common name of: (a) methanoic acid, and (b) ethanoic acid?

Hint: In the given question it is asked about the common names of the given carboxylic acids. Even in the normal language they are referred to as acids. Now we can say that methanoic acid has one carbon atom whereas ethanoic acid has two and in general norms they are represented by formic and acetic respectively. That sums up most of the answer.

Complete step by step solution:
In the given question we have to find out the common names that are given for the compounds mentioned which are from the family of functional groups of carboxylic acids. They are generally denoted with a common reference of acids in the common nomenclature too so that will remain constant for any case in the given question.
Common nomenclature is the second recognized nomenclature by chemistry which isn’t governed by much rules. It is very distinctive to IUPAC nomenclature and is also widely used.
Now we have to take both the compounds one by one in order to analyse and find the right answer for their common names.
Methanoic acid:
The chemical formula of the methanoic acid is \[HCOOH\].
It is first in the carboxylic acid series.
Its common name is Formic acid, as formic is generally used for functional groups with one carbon atom.
Ethanoic acid:
The chemical formula of the ethanoic acid is $C{H_3} - COOH$.
It is second in the carboxylic acid series.
Its common name is Acetic acid, as acetic is generally used for functional groups with two carbon atoms.

Note: A carboxylic acid is an organic acid that contains a carboxyl group \[\left( {C\left( { = O} \right)OH} \right)\] attached to an R-group. The general formula of a carboxylic acid is \[R-COOH\], with R referring to the alkyl group. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.