The red blood cells of type AB blood have on their surfaces:

This means that the electrical activity that causes the atria to contract starts from the sinoatrial node and spreads across the atria.
The P wave on the ECG reflects this atrial depolarization.
Choice A is wrong because repolarization of atrial muscle fibers is not visible on the ECG, as it occurs during the QRS complex when the ventricular depolarization masks it.
Choice C is wrong because the depolarization of ventricular muscle fibers is represented by the QRS complex on the ECG, not the P wave.
Choice D is wrong because the repolarization of ventricular muscle fibers is represented by the T wave on the ECG, not the P wave.
Normal ranges for the P wave are:
Duration: less than 0.12 seconds (less than 3 small squares)
Amplitude: less than 2.5 mm (0.25 mV) in the limb leads, less than 1.5 mm (0.15 mV) in the precordial leads
Axis: between 0° and +75°12

This is because blood type AB has both A and B antigens on the surface of red blood cells.

Antigens are protein molecules that can trigger an immune response if they are foreign to the body.
Choice A is wrong because antigen B only is present in blood type B.
Choice B is wrong because neither antigens A nor B are present in blood type O.
Choice C is wrong because antigen A only is present in blood type A.
Normal ranges for blood types vary by population, but according to the NHS, about 3% of people in the UK have blood type AB.