Answer:
C: With thanks, I accept your kind invitation.
Explanation:
B and D are obviously not correct, they have simple grammatical issues: B- second sentence only implies a subject, D- not a full statement.
A and C are appropriate, but C is the answer because A uses a contraction, and just from reading it, C looks more formal.
Answer:
C - With thanks, I accept your kind invitation
Explanation:
Formal language is the type of language one would use when talking to someone you do not have a good relationship with. Options A and B are information (like you would talk to a friend) and Option D is brief and could be considered somewhat rude.