Atrial systole marks the end of ventricular diastole with a brief atrial contraction.

Steps:

• Identify cardiac cycle phases: atrial systole follows ventricular filling and precedes ventricular systole.
• Locate pressure traces: atrial pressure rises sharply while ventricular pressure shows a minor increase.
• Match arrow to timing: atrial systole aligns with the 'a' wave or small atrial peak before isovolumetric contraction.
• Confirm against cycles: arrow B points to this consistent feature across both cycles.

Why B is correct:

• Atrial systole is defined as atrial contraction that boosts ventricular end-diastolic volume, shown by a distinct atrial pressure spike per standard Wiggers diagram.

Why the others are wrong:

• A indicates ventricular diastole onset, with falling pressures.
• C marks ventricular systole start, with rapid ventricular pressure rise.
• D shows ventricular relaxation or early diastole, with declining pressures.

Final answer: B