Option C. 5/26
Let 104 kg of each be melted.
then copper in the first alloy = 104×5/8 = 65 kg.
Zinc in the first alloy = 104×3/8 = 39 kg.
Copper in 2nd alloy = 104×8/13 = 64 kg
Tin in 2nd alloy = 104×5/13 = 40 kg.
Total alloy = 104 + 104 = 208 kg.
Weight of tin per kg. = $\frac{40}{208}$ = $\frac{5}{26}$.