Answer
See explanation below.
Work Step by Step
When metal A reacts with metal B salt, solid colored product is formed on metal A.
Conclusion is that Product is solid metal B and A is better reducing agent than B.
When metal B reacts with acid gas bubbles are formed.
Conclusion is that H2 gas produced as result of reduction of H+ and B is better reducing agent than acid.
When metal A reacts with Metal C salt no reaction occurs.
Conclusion is that C should be a better reducing agent than A.
As C is a better reducing agent than A, a better reducing agent than B and B reduces acid, then C might also reduce acid to form H2 bubbles.
Hence the order of strength of reducing agents is: C > A > B.
