Remember that the work function \(\phi\) is an intrinsic property of a material and does not change due to variations in external conditions such as light frequency or intensity.
Step 1: Define the work function. The work function is the least energy needed to pull an electron out of a metal surface. It is written as $\phi$.
Step 2: See where it comes from. The work function depends on how tightly the metal holds its electrons. This is set by the metal's atomic structure, so it is a property of the metal itself.
Step 3: Check the frequency of light. The frequency of incident light decides whether an electron can be knocked out, but it does not change the metal's own work function. So frequency is not the answer.
Step 4: Check the intensity of light. Intensity sets how many photons arrive, which changes the number of electrons emitted, not the energy needed to free one. So intensity is not the answer.
Step 5: Check the velocity of light. The velocity of light in vacuum is a fixed constant and has nothing to do with the work function. So velocity is not the answer.
Step 6: State the answer. The work function depends only on the nature of the metal. \[ \boxed{\text{Nature of the metal}} \]