An incomplete combustion of methane among the following reactions is-

1. 2CH4+O2Cu/523 K/100 atm2CH3OH

2. CH4+O2Mo2O3HCHO+H2O

3. CH4+O2C+2H2O

4. CH4+2O2CO2+2H2O



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:

CH4(g)+O2(g)CombustionIncompleteC(s)+2H2O(l)