We are told that humans need love and friendship to be primary motivations to be happy- and that we can satisfy economic needs without love and friendship. The correct answer is essentially a must be true- it would be a conclusion to the set of statements already presented.
We cannot prove that A is the case- that people can only be happy when economic utility is a motivator. We do not know that economic utility being a motivator stops love and friendship from being motivators as well. We know we must have those things to be happy, but nowhere are we told that these motivations cannot coexist with economic utility as another motivator as well.