Pair production cannot take place in a vacuum because:

Explanation

Correct answer: C) Momentum is not conserved. The principle of conservation of momentum states that the total momentum of an isolated system remains constant in the absence of external forces. In pair production, when a photon is converted into an electron-positron pair, both the photon and the newly created particles (electron and positron) have momentum. For momentum to be conserved, the combined momentum of the electron-positron pair should be equal to the momentum of the original photon. This can only occur if there is another particle or system to absorb the excess momentum. In a vacuum, however, there are no other particles present to balance the momentum, making it impossible to conserve momentum during pair production. Therefore, this option is the correct answer.