The kinetic energy of the electron in an orbit of radius $r$ in hydrogen atom is proportional to ($e$ = electronic charge)

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

An electron in the hydrogen atom jumps from $n^{th}$ energy state to the ground state. The wavelength so emitted illuminates a photosensitive material having work function $2.65$ eV. If the maximum kinetic energy of the emitted photoelectrons is $10.1$ eV, then the value of '$n$' is