A & B are partners & they admit C as a new partner and C brings ₹18,000 out of his share of premium for the goodwill of ₹30,000. Assuming that goodwill does not exist in the books, which account is debited with this difference of ₹12,000?

C's Current A/c

The correct answer is option 2- C's Current A/c.

When the new partner does not bring the share of goodwill, and goodwill does not exist in the books, sacrificing partners are credited with their share of goodwill and new partner is debited by the amount of goodwill not brought by him.
The journal entry in this case is:
Incoming (New) Partners Current A/c Dr.
    To Sacrificing Partners Capital A/c (individually)
(Account of goodwill not brought in by new partner)

Here, C is the incoming partner so his current account is debited with ₹12,000.