Which component is most likely leaking if a technician notices fluid around the rear of the transmission?

When evaluating fluid leakage around the rear of the transmission, the output shaft seal is the most likely component responsible for this issue. The output shaft connects the transmission to the driveshaft, and when the seal that surrounds the output shaft becomes worn or damaged, transmission fluid can leak out, escaping from the rear of the unit.

The output shaft seal directly manages the containment of transmission fluid within the transmission housing. If it fails, it creates an opportunity for fluid to seep out, resulting in visible leakage at the rear of the transmission. This leaking effect can be significant, especially in vehicles where the transmission fluid level is critical for operation and can lead to transmission damage if not addressed promptly.

Other components listed, like the input shaft seal, shifter shaft O-ring, and speedometer housing O-ring, typically don't produce fluid at the rear of the transmission. The input shaft seal is situated at the front of the transmission, while the O-rings associated with the shifter and speedometer deal largely with control and signal functions rather than fluid containment. Thus, the context of the fluid's location makes the output shaft seal the most plausible origin of the leak.