How does a sample of helium at 15 °c compare to a sample of helium at 215 k?
a. the helium at 15 °c has a higher average kinetic energy than the sample at 215 k.
b. the helium at 15 °c has lower nuclear energy than the sample at 215 k.
c. the helium at 15 °c has slower-moving atoms than the sample at 215 k.
d. the helium at 15 °c has smaller atoms than the sample at 215 k.
i think the answer is a, but i'm not sure.

You are right, the correct choice is:

Explanation:

Start by converting 15 °C to K (kelvin) to compare the temperatures in the same basis:

Therefore, the temperature of the sample of helium at 15°C  is higher than the sample of helium at 215 K.

On the other hand, according to the kinetic theory of gases, the temperature of a gas is a measure of the average kinetic energy of the gas particles (atoms or molecules), and the higher the temperature the higher the average kinetic energy of the gas particles.

Hence, you conclude that the helium at 15 °C (288 K) has a higher average kinetic energy than the sample at 215 K.