What term refers to four letters that come together to make one sound?

The term that correctly describes four letters coming together to make one sound is "quadrigraph." This term specifically refers to a grouping of four letters that, when combined, produce a single phonetic sound, working much like a digraph (two letters) or trigraph (three letters). Notably, quadrigraphs often appear in certain language constructs and represent unique sounds that would not be phonetically deduced from each individual letter alone.

Other terms in the choices, such as trigraph, digraph, and monograph, serve different linguistic functions. Trigraphs involve three letters forming one sound, while digraphs consist of just two letters creating a single sound. Monograph, on the other hand, refers to a written work or treatise on a specific topic and is unrelated to phonetics. Understanding the distinctions among these terms is crucial for grasping the complexities of phonology and word formation in language education.