What is the numerical difference in molar masses of second and third member of a homologous series?

Identify alkadiene molecule from following.

Identify the reagent R used in following conversion?

tert-butyl bromide $\xrightarrow{\mathrm{R}}$ Isobutylene

Identify the correct stability order of following alkenes.

I) $\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_3\right)_2$

II) $\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{CH}_2$

III) $\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{CHCH}_3$