(D) Killer T-cells
The second line of defence involves non-specific cellular and molecular responses of the innate immune system because these defences do not differentiate between different types of the pathogen and respond the same way upon every infection. Mucous membrane, phagocytes and lysozyme are all involved in a non-specific defence mechanism, whereas, killer T-cells or cytotoxic T-cells provide the third line of defence.
So, the correct answer is 'Killer T-cells'.