Correct Answer: Dialect

Solution : The correct choice is the second option.

The correct one-word substitute for the given group of words is dialect. A dialect refers to a particular form of language that is specific to a particular region, social group, or community. It involves variations in vocabulary, grammar, pronunciation, and usage that distinguish it from other forms of the same language.

The meanings of the other options are as follows:

• Singular refers to something unique, exceptional, or one of its kind.
• Slang represents informal words, phrases, or language used in casual settings, often specific to particular social groups or contexts.
• Referendum refers to a general vote by the electorate on a single political question that has been referred to them for a direct decision.

Therefore, the correct answer is dialect.