Step-up transformers enable efficient long-distance power transmission by reducing current losses with AC.

Steps:

• Step-up transformers increase voltage output relative to input.
• For constant power (P = VI), higher voltage results in lower current.
• Lower current minimizes heat loss (I²R) in transmission cables.
• Transformers require changing magnetic fields, so they operate only on alternating current (a.c.), not direct current (d.c.).

Why C is correct:

• Step-up transformers decrease current to reduce energy losses in lines, per the power conservation formula P = VI, and function exclusively with a.c.

Why the others are wrong:

• A: Transformers do not work with d.c.; current is decreased, not increased.
• B: Current decreases, not increases; d.c. is incompatible.
• D: Purpose is to decrease current for efficiency, not increase it, even with a.c.

Final answer: C