The police car has priority because it is driving on a priority road, indicated by a yellow diamond sign (B1). In Dutch traffic rules, the general rule 'drivers from the right have priority' only applies when there are no traffic signs or markings that indicate otherwise. Priority roads override this rule. Even if the motorcycle is approaching from the right, the police car, being on the priority road, has the right of way and the motorcycle must give way.
This is a common confusion point. Always look for priority signs, yield signs, or road markings like shark's teeth (chevron markings at intersections). If there is a priority road sign, vehicles on that road have priority over all others, regardless of the direction from which other vehicles come. If the police car were an emergency vehicle with flashing lights and siren, it would also have priority, but that is a separate rule. In your theory test, always check for signs first.
Practical steps to master this: memorize the key priority signs: the yellow diamond (priority road), the yellow diamond with a diagonal line (end of priority road), the triangular yield sign, and the stop sign. Then, when you see an intersection question, spot the signs before applying any general rule. The hierarchy is clear: traffic signs override road markings, and road markings override the general 'right before left' rule. Honest tradeoff: many learners find it counterintuitive that a vehicle from the right does not always have priority, but once you link the signs with priority it becomes straightforward. Next, practice with online theory tests that show various intersections, and review the section on priority in your study material. Pay special attention to examples where priority roads cross normal roads, as those are frequent test items.