Answer
(a) $photon\ B$
(b) $666\ nm$
Work Step by Step
(a) We know that momentum and wavelength are inversely proportional. This means that the photon having higher momentum will have the smaller wavelength. Thus, photon B has the smaller momentum and larger wavelength.
(b) As $p_A=2p_B$
$p_A\times \lambda_A=p_B\times \lambda_B$
We plug in the known values to obtain:
$2p_B\times \lambda_A=p_B\times \lambda_B$
$2\times 333\times 10^{-9}=\lambda_B$
$\implies \lambda_B=666\times 10^{-9}m=666nm$
