Answer:
Rosa es la mayor.
Step-by-step explanation:
Susana tiene 19 años, Marta 21 y Rosa 25. Por lo tanto, Rosa es la mayor. If Susana is 19 years old, Marta is 21 and Rosa is 25, this means Rosa is the oldest. The superlative "la mejor" means "the best", which has no sense when talking about ages. It cannot be determined which of the girls is the best by just saying their age. "La peor" means "the worst", which also has no sense when referring to ages.