Which of the following statements is true?
a. processes are spontaneous because they occur at an observable rate.
b. processes that are spontaneous in one direction are spontaneous in the opposite direction.
c. spontaneity can depend on the temperature.
d. all of the statements are true.

C

Explanation:

A process is called spontaneous if the process takes place on its own without the intervention of external factors.

The spontaneous processes are generally quick with observable rates of reaction.But processes can be spontaneous even with negligible rates of reaction.

Spontaneity of reactions depend on temperature.

This is because spontaneity is measured by gibbs energy or enthalpy.

Both of these measures are dependent on temperature.

So,temperature affects spontaneity.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.