Answer
Humans and mice are endothermic/warm blooded, and so they have a similar body temperature around 37 degrees C. Clearly their lipids can move freely when the cells are fused, and this movement is passive, since inhibiting ATP/energy use makes no difference. What does inhibit movement is 4 degrees C, which would cause the membrane to freeze solid.
Work Step by Step
To figure out this question, first consider the course of the cells and the temperatures used--37 degrees C is normal for humans and, probably, for other warm-blooded mammals, such as mice. Then consider the non-effect of ATP--which is looking at energy use to move lipids. Energy is not needed, so movement is passive. Finally, lower temperature, which would freeze a membrane designed to work at warm body temperature, stops movement.
