Glycoproteins primarily enable cell recognition

Steps:

• Identify glycoproteins as proteins with carbohydrate chains on cell membranes.
• Recall their main role: acting as markers for cell-cell recognition and signaling.
• Evaluate functions: they do not transport ions/molecules (that's membrane proteins like channels) or provide structural support (that's lipids and cytoskeleton).
• Match to options: only function 2 (cell recognition) fits.

Why C is correct:

• Glycoproteins serve as identification tags for immune response and tissue formation, per cell biology definitions of membrane components.

Why the others are wrong:

• A includes 1 and 3, which are transport and structure—glycoproteins do not perform these.
• B includes 3 (structure), but glycoproteins lack structural roles.
• D selects only 3, ignoring the key recognition function.

Final answer: C