The Government of India Act of 1919 clearly defined:

The answer is B. The Government of India Act 1919 (Montagu-Chelmsford reforms) was the first statutory step towards separating central and provincial subjects. Section 45A allowed rules to be framed, and the Devolution Rules then divided subjects into central and provincial heads, with provinces getting a measure of responsibility under dyarchy.

• A is wrong: The Act did not separate the judiciary from the legislature. That separation was not its purpose, and the High Courts and legislative structures stayed largely as they were.
• C is wrong: The Act did not define the powers of the Secretary of State and Viceroy by the importance of the subject. Central control over the provinces remained strong.
• D is wrong because B is substantially correct among the options.

Core concept: fiscal and administrative devolution, and dyarchy (reserved and transferred subjects) in the provinces. The 1935 Act later introduced the three-list scheme (Federal, Provincial, Concurrent), which the Constitution inherited in the Seventh Schedule. For Mains, this traces the roots of Centre-State relations.

Trap: Candidates attribute the three-list federal scheme to 1919. That formal scheme belongs to the 1935 Act, and the 1919 division was operated largely through rules.