Correct answer is (b) 1.3 mol/L
For explanation: When two solutions are combined, the resultant molarity is M1V1 + M2V2/V1 + V2. Consider M1 = 1.3, M2 = 0.8, V1 = 100ml and V2 = 500ml. Now resultant molarity = 1.3(100) + 0.8(500)/100 + 500 = 0.88 mol/L.