On controlled hydrogenation of benzene with group I metal (like Na, Li) and methanol (or ethanol) in liquid $NH _{3}$, it reduces into cyclohexadiene and cyclohexene, which cannot be isolated
This is because the presence of reducing agent hexadiene is further reduced to hexene. hence,
statement (a) is correct.
Statement (b) is incorrect due to data insufficiency.
Statement (d) is incorrect because benzene is a stable aromatic compound while cycloohexene can be easily hydrogenated due reactive double bond. So correct choice is (a).
