At the bottom, deep bodies of water always measure 4°C, because at that temperature water has its highest [...] (fill in the physical quantity in question)

density

Explanation:

At the bottom, deep bodies of water always measure because at that temperature water has it highest density .

In water bodies like lake warm water rises up due to convection and at higher depths there is cold water, which is found to be at highest density of water.

When water molecules acquired heat energy  it become widely spread at the same volume and thus posses low density but at low temperature water molecule occupy less space therefore posses maximum density at