Concept:Use the ratio of gold to total weight before and after adding pure gold.Explanation:Gold in the original 50 g alloy is:70% of 50=10070​×50=35 gLet x g of pure gold be added to the alloy.New gold content =35+x gNew total weight =50+x gFor the new mixture to contain 85% gold:50+x35+x​=10085​Cross-multiply:100(35+x)=85(50+x)3500+100x=4250+85x100x−85x=4250−350015x=750x=50 gAnswer:The quantity of gold to be mixed is 50 g, i.e., Option B.