Correct answer: you must park in a permitted and safe place, keeping the position lights on.

If it is raining so hard that you cannot continue safely even at low speed with your lights on, find a place where parking is allowed and safe, and wait for the rain to ease with your position lights on so others can see you. Stopping right on the roadway is dangerous, because the driver behind may not see you in time. Speeding up with so little visibility is even more dangerous.

Cuestiones de Seguridad Vial. Manual II · Real Decreto 1428/2003, de 21 de noviembre, por el que se aprueba el Reglamento General de Circulación