Answer
Morphology
Explanation
Morphology is the study of word structure, how they form, and their relationships in a given language. Morphology tell us that words are made of smaller units called morphemes. A morpheme according to Zelig Harris is "Every sequence of phonemes which has meaning and which is not composed of smaller sequence having meaning, is a morpheme."
Morphemes combine hierarchically to form words; we usually use morphology threes to study the combination of morphemes into words.