The correct answer is C) to prevent excessive temperature rise. The purpose of a heat sink is to dissipate the heat generated by a transistor or other electronic component during operation. Transistors, being semiconductor devices, can generate significant heat as they switch and conduct current. If this heat is not effectively removed, the transistor can overheat and potentially fail. By attaching a heat sink, the heat generated by the transistor is transferred to the larger surface area of the heat sink, allowing it to dissipate the heat into the surrounding environment, thereby preventing excessive temperature rise in the transistor itself. The other answer choices are incorrect because they do not accurately describe the primary function of a heat sink. Increasing reverse current (A), compensating for excessive doping (B), and decreasing forward current (D) are not the main roles of a heat sink in an electronic circuit.