Answer: Ageing population
Explanation:
From the information given in the question, we are informed that Mariana is writing a report on Spain and that the percentage of people under age 25 is declining, while the percentage of the population that is over 50 is rising.
This shows that the above population is an ageing population. An ageing population is a kind of population where the adults in the society are increasing more than the number of younger people in the society.
