Which of the following is an example of an idiosyncratic rule?

An example of an idiosyncratic rule is phonological variations in dialects because idiosyncratic rules refer to specific, often unique patterns of language use that can vary greatly from one speaker or dialect to another. In this context, phonological variations represent how different groups of speakers may alter sounds in ways that are particular to their regional or social dialect. These variations may not follow the common phonological rules of the standard language, making them unique to those speakers.

Common grammar rules in a language generally reflect widespread conventions that speakers of that language adhere to, while universal sound patterns refer to features that are consistent across numerous languages. Cross-linguistic word meanings involve how words are interpreted in different languages and don't necessarily illustrate the personal or localized nature of idiosyncratic rules. Thus, phonological variations stand out as the best representation of an idiosyncratic rule among the choices provided.