A a variant of a phoneme that phonetically differs from one another. Their occurrences are usually determined by their position in the word (initial, final, medial, etc.) or by their phonetic environment, not changing the meaning of the word.
B the smallest unit of speech distinguishing one word (or a word element) from another, changing the meaning of the words. The phonetic environment is not important.
C the phoneme variation that, besides changing the meaning of a word, increases vocabulary. This variation is extremely important to show the hierarchy among speakers.
D the core part of a phoneme that, if modified, changes the meaning of a word. Once you change it, speakers notice the variation promoted by age, origin, accent, etc.