Human beings can live happily only in a society where love and friendship are the primary motives for actions.
Background
conditional (only)
Live happily → Love/friendship are primary motives for actions
Love/friendship are not → can't live happily primary motives for actions
Pivot (Yet) Economic needs can be satisfied even when love / friendship are not the primary motives for actions.
Setup
So, human beings ....
Evaluate
Usually with Logical Completion (i.e. fill in the blank) questions, it's pretty doable to predict where the fill in the blank is going, because we'll have two different strands of thought that we are going to try to safely bring together.
In this case, it's a little weird. The Pivot word suggests that we should be trying to figure out how to reconcile the 1st and 2nd sentence. It sounds like Happiness and Economic Needs are two different things.
Happiness requires love and friendship being the primary motives for action whereas economic needs can be satisfied even if economic utility is all that motivates action.
Goal
Most wrong answers on Logical Completion are too strongly worded. So, given that we don't have much of a prediction, let's just look for something safely worded that balances the differing needs of "living happily" and "satisfying economic needs".