The Pitt’s India Act of 1784 was significant because it:

The correct option is Distinguished between the commercial and political functions of the Company.

Step-by-Step Explanation:

1. Context of the Pitt's India Act of 1784:
The Pitt’s India Act (named after William Pitt the Younger, the Prime Minister of Great Britain) was passed by the British Parliament to rectify the deficiencies of the Regulating Act of 1773 and establish greater control over the British East India Company's administration in India.

2. Dual System of Control:
The most defining feature of the Act was that it clearly separated the commercial functions of the East India Company from its political duties:
- Commercial Functions: Managed exclusively by the Court of Directors of the Company.
- Political Functions: Placed under a newly created body called the Board of Control, which directly represented the British government.
This established a dual system of governance for British possessions in India.

3. Why the other options are incorrect:
- Established the Supreme Court in Calcutta: The Supreme Court at Calcutta was established by the Regulating Act of 1773 (functioning from 1774).
- Ended the Company's monopoly over tea trade: The Company's monopoly over tea trade (and trade with China) was abolished by the Charter Act of 1833.
- Introduced the Portfolio system: The Portfolio system was officially introduced by Lord Canning through the Indian Councils Act of 1861.