The main reason for the discontent among Indian soldiers leading to the Revolt of 1857 was the introduction of the Enfield Rifles with greased cartridges. The cartridges were rumored to be greased with cow and pig fat, which deeply offended both Hindu and Muslim soldiers, as cows are sacred to Hindus and pigs are considered unclean by Muslims. This led to widespread resentment and played a key role in sparking the revolt.
The other options, such as the Doctrine of Lapse and the Subsidiary Alliance, contributed to British dominance in India but did not directly provoke the discontent among soldiers that led to the 1857 rebellion.