Which part in the drivetrain connects the transmission to the rear differential?

The driveshaft is the correct choice because it serves as the critical component that connects the transmission to the rear differential in a vehicle's drivetrain. When the engine generates power, that power travels through the transmission, which then sends it down the driveshaft. The driveshaft transmits the rotational force from the transmission to the rear differential, allowing the rear wheels to turn and propel the vehicle forward.

This component is designed to handle the torque and speed requirements of the drivetrain, making it crucial for effective power transfer. In vehicles with rear-wheel drive, the driveshaft is usually a long, tubular shaft that must be balanced to minimize vibrations and ensure smooth operation. Its role is essential, as it efficiently connects different parts of the drivetrain that are typically not in direct proximity to each other.

In contrast, other options like the camshaft and crankshaft are parts that are more involved with the engine's internal operation and do not play a direct role in connecting the transmission to the differential. The mainshaft, while part of the transmission and involved in the gear mechanism for power delivery, does not extend to the rear differential. Hence, the driveshaft is the key component in this connection within the context of a vehicle's drivetrain.