Why does the inlet valve open before T.D.C. during the exhaust stroke?

The correct response indicates that opening the inlet valve before Top Dead Center (T.D.C.) during the exhaust stroke facilitates the induction of a greater amount of the air-fuel mixture into the cylinder. This process is known as "valve overlap." By retracting the inlet valve early, the engine allows some of the incoming mixture to enter the combustion chamber even while the exhaust gases are still being expelled.

This timing enhances the fresh charge's ability to enter the cylinder as the piston approaches T.D.C., ensuring efficient filling of the cylinder for the next power stroke. The momentum of the mixture being drawn into the cylinder can help to scavenge residual exhaust gases remaining from the previous cycle, ultimately leading to improved combustion efficiency.

The other options, while they may have some relevance in engine operation, do not directly correlate to the key functional purpose served by the early opening of the inlet valve during the exhaust stroke. Increase of pressure, reduction of vibration, and mixing of gases do occur as part of general engine dynamics but are not the primary rationale for the valve timing strategy in this specific context. The primary objective is to maximize the cylinder's intake efficiency and optimize engine performance by ensuring the fresh mixture is adequately introduced into the combustion chamber.