d
\({{\text{C}}_{{{\text{P}}_{{\text{mix}}}}}} = \frac{{{\mu _1}{C_{{P_1}}} + {\mu _2}{C_{{P_2}}} + {\mu _3}{C_{{P_3}}}}}{{{\mu _1} + {\mu _2} + {\mu _3}}}\,\,\, = \frac{{4 \times \frac{{7R}}{2} + 2 \times \frac{{5R}}{2} + 1 \times \frac{{8R}}{2}}}{7} = \frac{{23R}}{7}\)