In a typical medium speed 4-stroke cycle diesel engine the inlet valve
A. Opens at $20{}^\circ $ before top dead center and closes at $35{}^\circ $ after the bottom dead center.
B. Opens at top dead center and closes at bottom dead center.
C. Opens at $10{}^\circ $ after top dead centre and closes at $20{}^\circ $ before the bottom dead center.
D. May open or close anywhere

Hint: As a very first step, one could read the question very carefully along with the options to understand what exactly we are supposed to find. Then, you could give a brief description on the inlet valves and also their function. Then, you could find which among the given options is true for the specified engine.

Complete step-by-step solution:
In the question, we are enquired about the inlet valve of a diesel engine. We are specified that the diesel engine is a typical medium speed 4stroke cycle diesel engine.
You may recall that the inlet valves which are otherwise called intake valves are opened to allow the air/fuel mixture flow into the engine’s cylinders before compression and ignition. So, basically these are the valves through which a fluid is drawn inside of an engine. Induction valve is another name given to these valves.
Now, let us come back to the question. It is a well known fact that for a typical medium speed 4stroke cycle diesel engine, the inlet valves open at $20{}^\circ C$ before the top dead centre and closes at $35{}^\circ $ after the bottom dead centre.
Hence, option A is found to be the answer.

Note: We also have another category of valves called the exhaust valves. Unlike the inlet valves, the exhaust valves let the exhaust air out. Also, we could determine the power of the engine from the movement of air. The more the air that moves in and out of the engine, the more efficient and hence more the power an engine has.