Which of the following is not linked with methane?

The octane number of $2, 2, 3-$trimethylbutane is considered to be ............ .

In the reaction of $1$-bromo-$3$-chlorocyclobutane with two equivalents of sodium in ether,the major product is

On mixing an alkane with chlorine and irradiating with ultra-violet light, it forms only one mono-chloro-alkane. The alkane is

$CH_3-CH(CH_3)-CH_2-CH_3 \xrightarrow[hv]{Cl_2} (x)$. Find the number of monochloro products including stereoisomers.

Explain: Alkyl group.