Cytoplasmic extract from the wild type strain of a bacterium has the ability to convert a colorless substrate (S) to a colored product (P) via three colorless intermediates X, Y and Z, in that order. Each step of the pathway involves a specific enzyme coded by a distinct gene. Four mutant strains (a–, b–, c–, d–) were isolated, whose extracts are incapable of producing the colored product in the presence of S. In a series of experiments, extracts from the individual mutants were incubated with X, Y or Z and secored for color development. The data are summarized in the table below. (Yes : color developed, No : no color developed)
Based on the data, which one of the following  is the correct order of enzymes involved in the pathway ?