In a CB amplifier the maximum efficiency could be

Answer: Option C

Option A: 99%
An efficiency of 99% is highly unlikely for a Common Base (CB) amplifier. Such a high efficiency is typically not achievable in practical amplifier circuits due to inherent losses in the components and design. Therefore, this option is incorrect.

Option B: 85%
An efficiency of 85% is possible for certain types of amplifiers like Class C amplifiers, which are specifically designed for high efficiency. However, the CB amplifier operates in Class A mode, which inherently limits its maximum efficiency. Hence, this option is incorrect.

Option C: 50%
For a CB amplifier operating in Class A mode, the theoretical maximum efficiency is 50%. This is due to the symmetrical operation of the circuit, where half the power is dissipated as heat in the transistor. Therefore, this option is correct.

Option D: 25%
An efficiency of 25% is too low for a CB amplifier operating in Class A mode. While practical losses can reduce efficiency, it typically does not drop this low under normal operating conditions. Hence, this option is incorrect.

Conclusion:
The maximum efficiency of a CB amplifier could be 50% (Option C).