The IUPAC name of the simplest aldehyde, $\mathrm{HCHO}$, is

1. NCERT §8.1 (Nomenclature): the IUPAC name of an aldehyde is formed by replacing the '-e' of the parent alkane with '-al'. 2. $\mathrm{HCHO}$ has ONE carbon atom (the parent alkane is methane). Adding the '-al' suffix gives METHANAL. 3. Common name: formaldehyde. 4. Distractors: methanol (option B) is $\mathrm{CH_3OH}$, an alcohol. Methanoic acid (option C) is $\mathrm{HCOOH}$, the carboxylic acid form. Ethanal (option D) is $\mathrm{CH_3CHO}$, the TWO-carbon aldehyde. _Source: NCERT Class 12 Chemistry Part 2, Ch 8 "Aldehydes, Ketones and Carboxylic Acids", §8.1.1 (Nomenclature, IUPAC rules), p. 1–2._