The correct answer is: They usually have one or more helical regions spanning the membrane bilayer
Transmembrane proteins are type of integral proteins meaning that they are permanently attached to biological membrane. Transmembrane proteins span through the plasma membrane so most of their structure is hydrophobic.
The major role of these proteins is transport role: they act as a channels that undergo conformational change in order to move a substance through the membrane.
Classified by their structure they can be put into two types: alpha helical and beta barrels.