Final answer:
The combining form is the combination of two roots and a combining vowel, used to create compound words.
Step-by-step explanation:
The correct answer to the question is C. The element known as the combining form is the combination of two roots and a combining vowel. This combining form is used to create compound words by joining separate elements together. For example, in the word 'blackbird', the combining form 'black' is combined with the root 'bird' to form a new word.
Similarly, in the word 'dogcatcher', the combining form 'dog' is combined with the root 'catcher' to create a compound word. The combining form provides the foundation of the word, while the roots contribute to the overall meaning.
It's important to note that the combining form can appear at the beginning, middle, or end of a word, depending on the specific word being formed.