Which statement about transmembrane proteins is true? a. they usually have one or more α helices spanning the membrane bilayer. b. they are always inserted with the amino terminus on the lumenal side and the carboxyl terminus in the cytosol. c. they cross the membrane only once. d. the signal sequences are always cleaved off.

The correct answer is option A. "They usually have one or more α helices spanning the membrane bilayer".

Explanation:

Transmembrane proteins usually have one or more α helices spanning the membrane bilayer. This structural feature allows that certain regions of transmembrane proteins to be embedded within the lipid bilayer. This is explained by the fact that α helices have all backbone hydrogen-bonds internally, therefore no polar groups are exposed and it could rest on the lipid bilayer easily.