What is the term for letters in English words that connect two morphemes?

The term that accurately refers to letters in English words that connect two morphemes is "affix." An affix is a morpheme that is attached to a root or base word to modify its meaning or create a new word. Affixes can be prefixes, which are added to the beginning of a word, or suffixes, which are added to the end. They serve a crucial role in word formation and help in understanding the grammatical functions of words.

The other terms listed do not fit the definition provided in the question. "Connective" is not a recognized linguistic term in this context. "Morpheme" is the smallest unit of meaning in a language and can stand alone or combine with other morphemes but does not specifically refer to letters that connect two morphemes. "Compound" refers to a word that is made up of two or more independent words combined together, rather than a term that connects morphemes.