Concept:Uncountable nouns refer to things that cannot be counted individually and do not have a plural form. They are always singular.Rule / Clue:If a noun names a substance, mass, or abstract idea, it is usually uncountable. “Butter” is a substance, so it has no plural.Explanation:Butter is an uncountable noun. We never say “butters”. The other options are countable and have plural forms.- Tooth → teeth- Ring → rings- Patient → patientsOnly butter remains singular in all contexts (e.g., “I need butter”).Answer:A. Butter