a
(a) See the following force diagram. Vehicle is moving down the frictionless inclined surface so, it's acceleration is \(g\sin \theta \). Since vehicle is accelerating, a pseudo force \(m(g\sin \theta )\) will act on bob of pendulum which cancel the \(\sin \theta \) component of weight of the bob.

Hence net force on the bob is \(F_{net}  = mg\cos \theta \) or net acceleration of the bob is \({g_{eff}} = g\cos \theta \)

\(\therefore \) Time period \(T = 2\pi \sqrt {\frac{l}{{{g_{eff}}}}} = 2\pi \sqrt {\frac{l}{{g\cos \theta }}} \)