Final answer:
The Employer Identification Number (EIN) used in HIPAA transactions is assigned by the Internal Revenue Service (IRS).
Step-by-step explanation:
The correct answer is c) Internal Revenue Service (IRS). The Employer Identification Number (EIN) is assigned by the Internal Revenue Service (IRS) to identify employers for tax purposes. In the context of HIPAA, the EIN is used in standard transactions to identify the employer of an individual described in a transaction.