(a) Tl is more stable in +1 oxidation state, then +3 oxidation state, therefore Tl3+ salts are oxidising agents. (b) Ga+−2e−⇒Ga3+ (More stable) ∴Ga+is a reducing agent. (c) Pb4++2e−⇒Pb2+ (More stable) Pb4+ is an oxidising agent. (d) As5++2e−⇒As3+ (Less stable) Thus, As5+ cannot be reduced so As5+ salts are not a good oxidising agent. An oxidising agent is defined as substance whose oxidation number decreases while reducing agent defined as substance whose oxidation number increases.