<span> cells maintain homeostasis as they are living things. The main way cells maintain homeostasis is through their cell membranes. Cell membranes protect the cell from the outside world, if there is too much of something in the cell, the cell membrane uses pockets to expel some of it, it trasports nutrients across the cell, and it also dictates what goes in and out of the cell. Those are some ways a cell maintains homeostasis. And all living things must maintain homeostasis</span><span>: <span>Regulate intracellular ion control (e.g. maintain very specific [Ca^2+] levels)
Regulate extracelluar to intracellular ion transport. E.g. for maintaining water and blood pressure via the kidneys.
Regulate metabolism, for temperature control (e.g. in brown adipose tissue).
Regulate transcription and translation, for hormone release and
</span></span>
I'd have to say d. the hand, because every human (for the most part, no offence intended to anyone) has a hand and is able to make good (or bad, rather) use of it. It's what a person always has access to at any given time or place, even if they don't have access to one of the other options.
