The correct option is B perimeter of the orbit
According to the expression given by de-Broglie for the wavelength of a particle of momentum mv
λ=hmv -------(1)
According to the Bohr's postulate, electrons move only in those orbits where angular momentum is quantised.
mvr=nh2π ---------(2)
Putting the expression for mv from equation 1 to equation 2,
we get nλ=2πr
For first orbit n=1
Hence λ=2πr
which is perimeter of the orbit.