Final answer:
Retroviruses are composed of an outer membrane that is a lipid bilayer, derived from the host cell membrane, and contain glycoproteins that facilitate attachment to host cells.
Step-by-step explanation:
Retroviruses, such as HIV, are composed of an outer membrane that is actually a lipid bilayer.
This envelope is derived from the host cell membrane as the virus buds off and it is embedded with viral proteins including glycoproteins which are crucial for the virus to attach to and enter host cells.
In the case of the provided options, the correct answer would be 'c) Lipid bilayer' since this is what constitutes the outer membrane of a retrovirus.
Glycoproteins such as gp120 and gp41 present in HIV enable the virus to bind to target cells and are coded for by the viral genome.