Both Assertion (A) and Reason (R) are true, and R is the correct explanation of A.

Phenyl is indeed used as a household germicide, and the reason behind its effectiveness as a germicide is that it is a phenol derivative. Phenol is a compound that has antiseptic and disinfectant properties, which make it effective against various microorganisms, including bacteria and fungi. Phenyl, being a derivative of phenol, inherits these germicidal properties.

Phenol works as a germicide by denaturing proteins and disrupting the cell membranes of microorganisms. It has the ability to penetrate the cell walls of bacteria and disrupt their metabolic processes, leading to their destruction. This makes phenol and its derivatives, such as phenyl, effective in killing and preventing the growth of harmful microorganisms.

Therefore, phenyl, being a phenol derivative, exhibits germicidal properties and can be used as a household germicide.