Why might the number of phonemes exceed the number of letters in the alphabet?

The option regarding differences in regional pronunciation and dialect is correct because phonemes are the distinct units of sound in a language, and their realization can vary significantly across different regions and dialects. For example, the same letter combinations can be pronounced differently in various accents or dialects, leading to an increase in the number of phonemes represented within a given set of words.

This phenomenon illustrates how sounds can be influenced by geographical and social factors, meaning that the same letters can correspond to different pronunciations based on a speaker's background. Some dialects may have additional sounds that aren’t present in the standard form of the language, thereby resulting in more phonemes than the alphabet has letters.

In contrast, while silent letters and variations in spelling may contribute to complexity in language, they do not inherently account for an increase in the number of phonemes themselves. The introduction of new words could contribute to a broader vocabulary but does not directly affect the fundamental relationship between phonemes and letters in the alphabet. Thus, the impact of dialectal differences on phoneme quantity makes the chosen answer robust in understanding the nuances of language sound systems.