D

Step 1: Understand that Planck's constant (denoted as $h$) relates the energy of a photon to the frequency of its associated electromagnetic wave through the equation $E = h u$, where $E$ is energy and $ u$ is frequency.
Step 2: Analyze the units involved: energy is measured in Joules (J), and frequency is measured in hertz (Hz), where 1 Hz = 1/s.
Step 3: Rearranging the equation gives us $h = \frac{E}{\nu}$. Plugging in the units, we get the units of Planck's constant as:

• Energy (J) = Joules (J)
• Frequency (Hz) = 1/s

Therefore, the units of Planck's constant become:
$h = \frac{J}{1/s} = J \cdot s$ (Joules multiplied by seconds).
Conclusion: The correct unit of Planck's constant is Joule-seconds (J·s). Therefore, the answer is D.