Final answer:
The intern's misunderstanding of the word 'post' due to different meanings in British and American English is an error of semantics (d).
Step-by-step explanation:
The error made by the intern from the United Kingdom can be attributed to semantics (d), which refers to the meanings of words and phrases in a particular language. The word 'post' in British English commonly means 'mail', whereas in American English it is often used to mean 'putting something on a bulletin board'. This confusion occurred because the same word has different meanings in different dialects of English, leading to a misunderstanding in cross-cultural workplace communication.