What setting of the reverser may cause the inability to drive a double-carriage tram?

The reverser, i.e., the direction selector, must be set to the same position in both carriages to enable movement. The situation presented in option B, where the reverser in the first carriage is set to "P" (Forward) and in the second to "T" (Rear), causes each carriage to attempt to move in opposite directions. As a result, the acting forces cancel each other out, preventing the entire train set from moving.