( A ) de-Broglie's wavelength is given by $\lambda = \frac{h}{mv}$ where m = mass of the particle, v = group velocity of the particle.
( B ) Half filled and fully filled orbitals have greater stability due to greater exchange energy, greater symmetry and more balanced arrangement
( C ) The uncertainty principle is $\Delta E \times \Delta t \geq \frac{h}{4 \pi}$
( D ) The energy of 2s orbital is less than the energy of 2p orbital in case of Hydrogen like atoms
