The correct option is C 0.888 M
The number of moles of Ba(OH)2=20×342100×171=0.4
Moles of OH− ions=0.4×2=0.8
The number of moles of HNO3=1.2×2=2.4
Hence, the final solution is acidic due to the presence of excess H+
H+=(2.4−0.8)×10001200+3420.57=0.888M