The word "which" in "nice," "six," and "it" has a different pronunciation. In "nice," "which" is pronounced as /wɪtʃ/, while in "six," it is pronounced as /sɪks/, and in "it," it is pronounced as /ɪt/. This difference arises due to the phonetic rules and the context in which the word is used.

The pronunciation of "which" in "nice" is influenced by the "ch" sound, which is a voiceless alveolar affricate. This sound is produced by briefly stopping the airflow and then releasing it with a puff of air. In contrast, the "six" pronunciation of "which" is a result of the "s" sound, which is a voiceless alveolar sibilant. The "s" sound is produced by forcing air out through the teeth while keeping the tongue blade close to the ridge behind the upper teeth.

In the word "it," the pronunciation of "which" is simplified to /ɪt/, which is a short vowel sound. This simplification occurs because "it" is a pronoun and often serves as a filler word in sentences, where the specific meaning of "which" is not crucial.

Data from the American English Pronunciation Dictionary by Merriam-Webster indicates that the pronunciation of "which" in "nice" is /wɪtʃ/, in "six" is /sɪks/, and in "it" is /ɪt/. The dictionary provides a comprehensive list of words and their pronunciations, which helps to clarify the differences in pronunciation for English speakers.

The variation in pronunciation is a common feature of the English language, where the same word can have different pronunciations depending on its context and the specific dialect being used. This phenomenon is known as phonological variation and is a result of the complex and evolving nature of language.