Right answer:
3. Carla es más alta que Cristina.
We often use adjectives to compare things, people, events, ideas, among others. This is used to compare nouns in three ways, comparisons of equality, comparisons of inequality, and in superlatives. In this problem, we need to use comparisons of inequality. These comparisons just as mas que conveys the idea of more than or menos que conveys the idea of less than. Since Carla is taller than Cristina, therefore:
Carla es más alta que Cristina
