a b c d

d

$\text{Energy supplied during charging } P = VIt = 15\times 10 \times 8 \times 3600 = 432\times 10^4 J$ $\text{Energy supplied during discharging } P = VIt = 14\times 5 \times 15 \times 3600 = 378\times 10^4 J$ $ \text{Efficiency } \eta = \frac{378}{432}\times 100 = 87.5$%.