Question

what is the main responsibility of protein channels attached to the cell membrane?
keeping cell turgidity at normal levels for homeostasis
extra support for the cell exterior and mechanical stress
allowing specific molecules across the cell membrane

Explanation:

Protein channels in the cell membrane are involved in facilitating the movement of specific molecules across the membrane. This is a key aspect of their function. Cell turgidity (related to osmotic balance and plant cell structure) and extra support for the cell exterior (more related to cell wall - like structures in some cells) are not the main role of protein channels. Protein channels act as selective passageways.

Answer:

allowing specific molecules across the cell membrane