I would say a letter as it represents a speech sound and is part of an alphabet. For example the letter "u" can be a 'you' sound or an 'uh' sound depending on whether it is in a word like "universal" or in say the word "but"
Explanation:
That is still k. The speech noise k is in the elements written ask but in the phonetic alphabet, according to the IPA, it is still k. Spelling a letter based on the alphabet and writing it in the universal phonetic characters is often another, but for it is equal.
