Answer:
The correct answer is option "a" intensive
Step-by-step explanation:
Words such as "extremely" are useful to make adjectives stronge, in this case, the adjective "busy".
This type of words are called "intensifiers" so the correct answer is A.
Options B and D are incorrect since "extremely" is not a noun or verb.
Option C is also incorrect since empty words are those words that do not add meaning to a sentence.