The unit of Planck's constant is

The correct option is Joule- s.

To find the unit of Planck's constant, we can use the quantum mechanical relation between the energy of a photon and its frequency, known as the Planck-Einstein relation:

$E=h\nu$

Where:
- $E$ represents the energy of the photon, measured in Joules (J).
- $\nu$ represents the frequency of the radiation, measured in Hertz (Hz) or per second ($s^{-1}$).
- $h$ is Planck's constant.

By rearranging the equation to solve for Planck's constant ($h$), we get:

$h=\frac{E}{\nu }$

Now, substituting the units of energy and frequency into this formula:

$\text{Unit of}h=\frac{\text{Joule}}{{\text{s}}^{-1}}=\text{Joule}\cdot \text{s}$

Therefore, the unit of Planck's constant is Joule-second (written as Joule- s).