'Adverb' is a word that is used to change or qualify the meaning of an adjective, a verb, a clause or another adverb.
In the given sentence, the verb is 'get' and 'automatic' is modifying the same.
Option C - 'automatically' is an adverb which describes the verb 'get'.
On the other hand, Option A - 'automatic' is an adjective which can modify a noun, but not the verb.
Option B - 'automated' is verb form in its past tense, which cannot modify the verb.
Therefore, the suitable option to complete the given sentence is : Option C - 'automatically'.
The completed sentence, thus, becomes : If the work is not done, you get your money back automatically.