The productivity of C4 plants is more than C3 plants because:

The correct answer is: They lack photorespiration

In C4 photosynthesis photorespiration is reduced by concentrating rubisco with CO2 to promote more carboxylation thereby reducing oxygenation. This is performed efficiently as the C4 pathway happens in two parts connected compartments. The reduced/lack of photorespiration gives them an advantage over C3 plants to survive in drought, hot climates, and even in limited nitrogen and carbon dioxide availability.
Thus, C4 plants have better productivity because C4 plants lack photorespiration.