The correct answer is: B. To gain possession of the wealth of India.
Mahmud of Ghazni was a 11th-century Turkic conqueror who invaded India 17 times. His primary motive was to plunder the wealth of India, particularly its gold and jewels. He also sought to spread Islam, but this was a secondary motive.
Option A is incorrect because Mahmud of Ghazni did not have a strong religious motivation. He was not particularly interested in converting Hindus to Islam, and he did not destroy Hindu temples for religious reasons.
Option C is incorrect because Mahmud of Ghazni was not particularly successful in spreading Islam in India. He converted a few Hindus to Islam, but the vast majority of Indians remained Hindu.
Option D is incorrect because Mahmud of Ghazni did not establish a Muslim state in India. He was a conqueror, not a ruler. He did not attempt to establish a permanent presence in India, and he returned to Ghazni after each invasion.
In conclusion, the most important cause of the invasion of Mahmud of Ghazni was to gain possession of the wealth of India.