Final answer:
Adam's writing of 'sed' instead of 'said' can be classified as a spelling mistake, which is typical for young learners. It exemplifies the challenges of English phonetic inconsistency, where the same sound can be represented by different letters.
Step-by-step explanation:
When five-year-old Adam wrote 'my mom sed no', the correct way to describe his spelling of 'sed' for 'said' is a spelling mistake. Adam is demonstrating an understanding of phonetics by sounding out the word 'said' and spelling it as 'sed', but he has not learned the conventional spelling of the word. This is a common occurrence with young children who are still learning to write and spell. In English, different letters and combinations can represent the same sounds, and memorizing these exceptions is often necessary as English is not a phonetically consistent language. For example, the sound [s] can be spelled as 's', 'c', or 'ss' depending on the word and its position, such as in 'asked', where [s] is spelled as 's', or 'century', where it is spelled as 'c'.