The key idea is extravascular compression: coronary vessels run inside the myocardium, so whatever pressure the surrounding chamber generates during systole squeezes them and limits flow.
Think about the left side first. The left coronary artery feeds the left ventricle, which generates aortic-level systolic pressure around $120\,\text{mmHg}$. That intense squeeze almost stops left coronary inflow during systole, so the left coronary bed fills chiefly when the ventricle relaxes — a strongly diastolic pattern.
Now the right side. The right coronary artery mostly supplies the right ventricle, a low-pressure chamber peaking near $25\,\text{mmHg}$ in systole. With so little compressive force, the artery is not meaningfully throttled, so it keeps receiving blood during systole and continues through diastole. The result is a biphasic tracing with flow in both phases, which is exactly what the "$d + s$" note describes.
There is a clinical corollary worth remembering. Because the left ventricle depends so heavily on diastolic filling of its coronaries, anything that shortens diastole — such as a fast heart rate (tachycardia) — preferentially reduces left coronary perfusion and predisposes to subendocardial ischaemia, since the subendocardium is squeezed hardest of all. The right coronary bed, sustained partly during systole, is comparatively less hostage to diastolic time. This is also why aortic diastolic pressure is the main driving force for left coronary flow.
Thus the contrast is: right coronary $=$ both phases (biphasic, low RV pressure); left coronary $=$ mainly diastole (high LV pressure). The options claiming a single phase for both, or the reversed assignment, contradict this physiology.
\[\boxed{\text{RCA: biphasic (systole + diastole); LCA: predominantly diastolic}}\]