Final answer:
Ions primarily cross the cell membrane through channel proteins as part of facilitated diffusion. Aquaporins are water channel proteins that in some cases can also transport ions.
Step-by-step explanation:
Ions are able to cross the cell membrane primarily by B) Moving through channel proteins. This process is known as facilitated diffusion, which allows ions to move down their concentration gradient through specific protein channels without the need for energy. While ions cannot move directly across the phospholipid bilayer because they are charged particles, they diffuse with the help of transport proteins like channel proteins and carrier proteins. Aquaporins are specialized channel proteins that mainly transport water, but some are also capable of co-facilitating the transport of ions along with water.