(a)
Alkylated alkenes are more stable. More the alkylation of alkene, more will be its stability.
$\therefore$ Order of stability of alkenes is
$R_{2}C = CR_{2} > R_{2}C = CHR > R_{2}C = CH_{2}$
$> RCH = CH_{2} > CH_{2} = CH_{2}$
$\therefore$ Tetra alkylated alkene is most stable.