Right option is (b) 0.79
The explanation is: The Pilling-Bedworth ratio is the ratio of the volume of the basic cell of a metal oxide to the volume of the basic cell of an equivalent or standard metal. This ratio is 0.41, 0.79, 1.71, and 2.03 for K2O, MgO, Cu2O, and Cr2O3 in that order.