Why did the government adopt a policy of Americanization

At the start of the twentieth century there were approximately 250,000 Native Americans in the USA – just 0.3 per cent of the population – most living on reservations where they exercised a limited degree of self-government. During the course of the nineteenth century they had been deprived of much of their land by forced removal westwards, by a succession of treaties (which were often not honoured by the white authorities) and by military defeat by the USA as it expanded its control over the American West.

a, c, & d are correct:

they wanted lands to settle the extra population.

they wanted new markets for english goods.

they wanted to avoid civil war.

the answer is use of support