A combination of letters in which the letters join together sound like the sounds they made in their own is called a

This is called a

a Minimal Pair is pairs of words or phrases in a particular language that differ in only one phonological element, such as a phoneme, toneme or chroneme, and have distinct meanings. They are used to demonstrate that two phones are two separate phonemes in the language.

a consonant blend (also called consonant clusters) are a collection of two or three different consonant sounds that are each heard when the word is pronounced

a schwa is the most common vowel sound. It is a reduced vowel in many unstressed syllables especially if syllabic consonants are not used.

and a diagraph is a double consonant (two consonants together) or a vowel and consonant combination pronounced as a single sound.

Your answer is C because if you read that definition it fits best.