The lac operon is an operon responsible for transport and metabolism of lactose in Escherichia coli and other bacteria. It has three structural genes. The lac Z gene codes for β-galactosidase which converts lactose into glucose and galactose. Thus, E. coli cells with a mutated Z gene of the lac operon cannot grow in medium containing only lactose as the source of energy because they cannot synthesize functional β-galactosidase. The lac Y gene that codes for β-galactoside permease which transports lactose into the cell; bacteria with mutated lac Y gene cannot transport lactose from the medium into the cell. The lac A gene codes for $\beta$-galactoside transacetylase. A constitutive mutant produces the gene product continually and lacks the control over its expression. In these mutants, the structural genes produced enzymes all the time in comparison to the wild type where the enzymes were synthesized only in the presence of lactose. Hence, they should have the mutation in a gene other than those responsible for the structural genes. E. coli cells prefer glucose over any other carbon source; in the presence of glucose, lac operon is switched off by catabolite repression which is not governed by lac Z gene. Thus, there is no effect of mutated lac Z gene on the choice of not using lactose in the presence of glucose. Option C is the correct answer.