Introduction:
Metals are known for their unique properties, including their electrical and thermal conductivity. One of the properties associated with metals is their temperature coefficient, which describes the change in their electrical resistance with temperature. In this question, we will discuss the true statement about the temperature coefficient of metals.

Explanation:
The temperature coefficient of a material is defined as the fractional change in its resistance per degree Celsius (or Kelvin) change in temperature. It indicates how the resistance of a material changes with temperature.

a) Metals have a positive temperature coefficient:
This statement is true. Most metals have a positive temperature coefficient, meaning that their resistance increases with an increase in temperature. This behavior can be explained by the movement of free electrons within the metal lattice. As the temperature rises, the lattice vibrates more vigorously, causing more collisions between the free electrons and the lattice ions. These collisions impede the flow of electrons, leading to an increase in resistance. Therefore, metals generally exhibit a positive temperature coefficient.

b) Metals have a negative temperature coefficient:
This statement is false. Metals do not typically have a negative temperature coefficient. Unlike certain semiconductors and insulators, which can have a negative temperature coefficient due to specific material properties, metals generally show a positive temperature coefficient.

c) Metals have zero temperature coefficient:
This statement is false. Metals do not have a zero temperature coefficient. The temperature coefficient of metals is positive, as discussed earlier. Therefore, the resistance of metals increases with increasing temperature.

d) Metals have infinite temperature coefficient:
This statement is false. Metals do not have an infinite temperature coefficient. The temperature coefficient of metals, although positive, is finite and specific to each metal. Different metals may have different temperature coefficients, depending on their crystal structure, electron mobility, and other factors.

Conclusion:
In summary, the true statement about the temperature coefficient of metals is that metals have a positive temperature coefficient. This means that their resistance increases with increasing temperature.