An incomplete combustion of methane among the following reactions is-

1.

2. ${\mathrm{CH}}_{4}+{\mathrm{O}}_{2}\stackrel{{\mathrm{Mo}}_{2}{\mathrm{O}}_{3}}{\to }\mathrm{HCHO}+{\mathrm{H}}_{2}\mathrm{O}$

3. ${\mathrm{CH}}_{4}+{\mathrm{O}}_{2}\to \mathrm{C}+2{\mathrm{H}}_{2}\mathrm{O}$

4. ${\mathrm{CH}}_{4}+2{\mathrm{O}}_{2}\to {\mathrm{CO}}_{2}+2{\mathrm{H}}_{2}\mathrm{O}$

HINT: Incomplete combustion forms carbon black.
EXPLANATION

During incomplete combustion of alkanes with insufficient amout of air or dioxygen carbon black is formed which is used in the manufature of ink, printer ink, black pigments and as filters. Thus,

The incomplete combustion of methane can result in a slew of partially oxidized products, including COCO, but also methanol, formic acid, formaldehyde, and higher hydrocarbons also.

However,the oxidation of methane to soot is generally given as the following:

${\mathrm{CH}}_{4}\left(\mathrm{g}\right)+{\mathrm{O}}_{2}\left(\mathrm{g}\right)\underset{\mathrm{Combustion}}{\overset{\mathrm{Incomplete}}{\to }}\mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)$