What property of a hormone would allow it to pass unassisted through a plasma membrane?

Ans.

Hormones are regulatory substances synthesized by various plants and animals and play role in proper growth, development, and maintain homeostasis. Hormones are released by signalling cells and show their actions after binding to specific receptors present on the surface or inside of the target cell.

Hormones, for which receptors are present inside the cells (in cytosol or nucleus) should be hydrophobic in nature. This is because plasma membrane is made up lipid molecules and small, hydrophobic, uncharged molecules can easily pass through this. For example, cortisol, estrogen, and progesterone hormones are lipid-derivatives and receptors for these hormones are present inside the cells.  

answer; (phospholipid);

a. by using fur or clothes to trap heat energy near the body