Final answer:
Adam demonstrated a spelling error by writing 'sed' instead of the correct spelling 'said'. This is a common phonetic spelling mistake for young learners like Adam.
Step-by-step explanation:
When five-year-old Adam wrote the sentence, "My mom sed no," replacing 'said' with 'sed', he demonstrated a spelling error. Learning to spell correctly is a process that evolves with age and education. At his developmental stage, it's common for children like Adam to use phonetic spelling, which means they write words based on how they sound rather than the correct spelling. While 'sed' and 'said' sound similar, the correct spelling is indeed 'said'. The example provided reflects an attempt to spell based on phonetic hearing, which is a very typical phase in a young child's language and writing development.