Answer:
a) Número de personas que suben a un autobús en una parada.
c) Conocer el ganador de la Liga de Campeones.
Explanation:
This problem is about random experiments.
Random experiments are defined as experiments where the outcome can't be predicted. To ensure that result, the subjects are selected randomly. So, in this case, the right answer must be a situation where subjects are randomly present.
People taking the bus is a random experiment, because there's no the exact subjects in a bus at any time, it happens randomly.
Also, the winner of a sport league is also a random experiment, because it happens after several games which cannot be predicted.
Therefore, the right answers are a and c.