Correct Answer:
C. to
The correct answer is to. The phrase 'look forward to' is a common idiom meaning to anticipate something with pleasure. In this idiom, 'to' functions as a preposition, and therefore it must be followed by a noun or a gerund (the -ing form of a verb), which 'buying' is in this sentence.
- with: 'Looking forward with' is not the correct preposition to complete this idiom and is grammatically incorrect.
- for: 'Looking forward for' is grammatically incorrect and does not form the established idiom.
- by: 'Looking forward by' is grammatically incorrect and does not fit the meaning of anticipating something with pleasure.