If the length and area of cross-section of a conductor are doubled, then its resistance will be doubled, unchanged, halved, or quadrupled?

The correct option is C unchanged
We know that the resistance of wire of length L and of cross section area A is
R = ρL/A
where ρ = resistivity of metal of the wire.
When length and cross section are doubled, the resistance becomes
R' = ρ(2L)/(2A) = ρL/A = R