The immediate cause that sparked the Revolt of 1857 was the introduction of:

The correct answer is: b) Greased cartridges for rifles.

The greased cartridges for rifles were made of cow and pig fat, which was offensive to both Hindus and Muslims. The cartridges had to be bitten open before loading, which caused outrage among the soldiers who were forced to use them. This was one of the immediate causes of the Revolt of 1857, also known as the Indian Rebellion of 1857.

The other options are incorrect because:

• Taxation on salt was a long-standing issue that had been causing resentment among the Indian people for many years. However, it was not the immediate cause of the Revolt of 1857.
• The Doctrine of Lapse was a policy that allowed the British East India Company to annex the territories of any Indian prince who died without a male heir. This policy was also unpopular among the Indian people, but it was not the immediate cause of the Revolt of 1857.
• Widow remarriage laws were introduced by the British in the early 1850s. These laws allowed widows to remarry, which was a radical change from the traditional Hindu practice of widowhood. However, these laws were not the immediate cause of the Revolt of 1857.