Which of the following is an integral multiple of $\dfrac{h}{2\pi}$ in Bohr's model of an hydrogen atom?

Activity of a radioactive sample decreases to $\left(\dfrac{1}{4}\right)^{th}$ of its original value in 4 days. Then in 16 days its activity will become $x$ times the original value. The value of $x$ is

In Boolean expression $A + B = Y$ implies that

The depletion region of p-n junction