The correct answer is: Chinese merchants have dominated trade in Tibet since 1950, when China invaded the country.
The verb “dominate” means to have control over something. The verb “invade” means to enter a country or territory illegally with an armed force. In this case, China invaded Tibet in 1950 and has since dominated trade in the country.
The other options are incorrect because they do not accurately reflect the historical events. Option A, “Chinese merchants were dominating trade in Tibet since 1950, when China was invading the country,” is incorrect because it implies that China was not in control of Tibet at the time. Option B, “Chinese merchants will have dominated trade in Tibet since 1950, when China has invaded the country,” is incorrect because it implies that China will not be
in control of Tibet in the future. Option C, “Chinese merchants will dominate trade in Tibet since 1950, when China had invaded the country,” is incorrect because it implies that China did not invade Tibet in 1950. Option D, “Chinese merchants had dominated trade in Tibet since 1950, when China will be invading the country,” is incorrect because it implies that China will invade Tibet in the future.