Right answer:
menos
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. The superlative is a form of an adjective that expresses the highest possible amount of something talking about what is the most or the least. So we must use the least formula here:
definite article (el, la, los, las) + menos + adjective
Hipopótamo means hippopotamus and jirafa means giraffe. Therefore:
****El hipopótamo es menos alto que la jirafa (The hippopotamus is less tall than the giraffe)****