Replacing all three methyl groups on the nitrogen of acetylcholine with larger alkyl groups would result in:

• A. An inactive cholinergic agonist; larger alkyl groups are too large for hydrophobic pockets
• B. An active muscarinic agonist
• C. A selective nicotinic antagonist
• D. No change in activity

Answer: (B)

The key idea is that the positively charged quaternary ammonium part of acetylcholine is what drives binding to cholinergic receptors, and how much steric bulk you can tolerate around nitrogen depends on the receptor subtype. If you replace the three methyl groups with larger alkyl groups, the molecule remains quaternary and still carries that positive charge, which is why it can still interact with receptors.

Muscarinic receptors have binding pockets that can accommodate some steric bulk around the ammonium group. The bulky analogue can still position the cationic center into the receptor and activate it, so you get agonist activity at muscarinic receptors, even though potency might change due to the larger substituents.

This wouldn't make it a nicotinic antagonist, because nicotinic receptors prefer smaller, less hindered quaternary ammoniums to fit into their binding site; the bulkier analogue would more likely lose nicotinic activity rather than antagonize it. And the effect isn’t zero change—the bulk alters potency, but receptor activation can remain, especially at muscarinic sites.