Bacteriophage switch between lytic and lysogenic life cycle depends upon the situation. The decision to start whether a lytic or lysogenic life cycle is made by the activity of the proteins encoded by the four genes cI, cII, cIII, and cro. Among them c class proteins are DNA (deoxyribonucleic acid) binding proteins.
Hence, the correct option is (a) cI.
Cro is a gene, not a protein.
Hence, the option (b) is incorrect.
Lac repressor is a protein that inhibits lac operon.
Hence, the option (c) is incorrect.
Lactose is an inducer of lac operon.
Hence, the option (d) is incorrect.