Which type of words are formed in a way where each letter corresponds to its most common sound?

The correct answer is indeed regular words. Regular words are typically those in which the way they are spelled directly corresponds to their phonetic pronunciation; that is, each letter generally represents its most common sound. This predictability allows readers to decode these words more easily. For example, the word "cat" is made up of three letters, each producing their standard sounds, leading to the expected pronunciation.

In contrast, irregular words do not follow standard phonetic rules and may contain letters that do not produce their common sounds, making them more challenging to read. Compound words consist of two or more words that come together to form a new meaning, but they may not adhere to phonetic consistency of individual sounds. Homophones are words that sound alike but have different meanings and spellings, which also deviates from the idea of letters correlating with their typical sounds. Therefore, regular words are the best fit for this question, as they exemplify the concept where each letter corresponds to its most common sound.