Right answer:
más económico que
The sentence viajar en el bus es más económico que alquilar un auto means traveling on the bus is cheaper than renting a car. So this sentence uses Comparison of inequality. Thus, menos que conveys the idea of less than. We can use a common formula called less formula as follows:
menos + adjective/adverb/noun + que
So económico is an adjective. Recall that an adjective is a word that gives us more information about a noun or pronoun.
