Refer to figure.
In a four-stroke piston engine, the opening and closing of the intake and exhaust valves do not coincide exactly with piston positions at TDC and BDC. Instead, valve timing is deliberately arranged so that valve events occur slightly before or after the theoretically ideal piston position, in order to optimise cylinder filling and emptying at normal operating RPM.

Valve lead refers to :

• Inlet valve opening BEFORE the piston reaches TDC at the end of the exhaust stroke. This gives the valve time to reach a useful opening area before the induction stroke begins, improving cylinder charge at high RPM. 
• Exhaust valve opening before BDC.

Without this lead, the valve would only start opening after the piston began its downward induction stroke, limiting the volume of charge that could enter the cylinder. 

Inlet valve opens before the piston reaches TDC → CORRECT. Valve lead is specifically the advance opening of the inlet valve before TDC. At the end of the exhaust stroke the piston is approaching TDC, and the inlet valve begins to open slightly before the piston reaches that position. This ensures the valve is already open and flowing when the piston begins its downward induction stroke, improving volumetric efficiency particularly at high RPM.

Exhaust valve opens 20° before the piston reaches BDC → INCORRECT. The usual value of valve lead for the exhaust valve is 50° or 60°, not 20°.

Inlet valve opens before the piston reaches BDC → INCORRECT. Valve lead refers to the inlet valve opening before TDC, not before BDC. 

Exhaust valve opens 50° before the piston reaches TDC → INCORRECT. Exhaust valve open before BDC, not TDC.