Final answer:
Not-for-profit accounting is a specialized field in accounting that focuses on the unique financial reporting and regulatory requirements of tax-exempt organizations.
Step-by-step explanation:
The specialized field in accounting among the given options is Not-for-profit accounting. Not-for-profit accounting focuses on the unique financial reporting and regulatory requirements of organizations that are exempt from income tax and operate for charitable, educational, religious, or similar purposes.
The other options in the question are not specialized fields in accounting. The Internal Revenue Service (IRS) is a government agency responsible for tax collection and enforcement. Certified public accountant (CPA) is a professional recognition for individuals who meet specific education, experience, and examination requirements to provide accounting services to the public. There is no such field as 'Math accounting'.