Option 1 is incorrect because while the assertion A is true indicating that a transformer is indeed useful for decreasing or increasing voltages, the reason R is not correct as transformers are not used in direct current (D.C.) circuits but in alternating current (A.C.) circuits.

Option 2 is also incorrect as the explanation of A is not accurate. Transformer does step up or step down voltage but it is used in A.C circuits, not D.C circuits.

Option 3 is the correct answer. The assertion A is true as transformers are indeed used for stepping up or stepping down voltages. However, the reason R is false as previously explained; transformers are not utilized in D.C. circuits.

Option 4 is not correct as the assertion A is indeed true. Transformers are indeed useful for stepping up or stepping down voltages. However, reason R is not accurate because transformers aren`t used in D.C. circuits.