C_4 -plants are more efficient in photosynthesis than C_3 -plants...

The $C_4$ pathway is certainly more efficient than the $C_3$ pathway in the sense of carbon fixation. In $C_3$ plants, carbon dioxide and oxygen are both present. This means that most of the ribulose-1,5-bisphosphate is carboxylated, but some of the ribulose-1,5-bisphosphate is oxygenated. This results in photorespiration. So the correct option is Lower rate of photorespiration.