Answer:
Diagraphs.
Step-by-step explanation:
Diagraphs are the combination of two letters, be they of a vowel or a consonant. These combined words then make a single sound when used in a word.
The given combinations of "ch, th, wh, sh" etc are examples of diagraphs. The two letters form a single sound making for a single production. Diagraphs are closely confused with blends for they are both combinations of letters. But their main difference is that diagraphs produce a single sound while blends produce the sounds of the individual letters.