Option 1 states that both statements in the question are individually true, and that Statement II is the correct explanation of Statement I.

Statement I states that the Communists left the AH India Trade Union Congress in 1931. This statement is true. The Communist Party of India, which was a separate entity from the Indian National Congress, withdrew from the trade union congress as they disagreed with its policies and believed it was not adequately addressing the concerns of the working class.

Statement II states that by 1928, the Communists were no longer working with the mainstream national movement. This statement is also true. By this time, the Communist Party had become more focused on their own revolutionary goals and strategies, and did not align themselves with the Indian National Congress. They pursued a more radical agenda and sought to establish a socialist society in India.

Option 1 suggests that Statement II is the correct explanation of Statement I. This means that the reason for the Communists leaving the trade union congress in 1931 was their decision to no longer work with the mainstream national movement by 1928. This explanation is reasonable as it shows a progression in the Communist Party`s shift in strategy and focus.

Therefore, the correct answer is option 1.