A pressure cooker works on the principle of elevating the boiling point of water by applying pressure.

Option 1 correctly explains the principle on which a pressure cooker operates. When pressure is applied to the cooker, the boiling point of water inside increases, allowing the food to cook at higher temperatures. This leads to faster cooking times and helps retain the nutrients in the food.

Option 2 is incorrect because a pressure cooker does not make food grains softer solely by applying pressure. It is the combination of elevated pressure and temperature that softens the food grains.

Option 3 is also incorrect because it states that a pressure cooker makes food grains softer only through the application of pressure and temperature. This is not entirely accurate as the primary principle behind a pressure cooker is the elevation of the boiling point of water.

Option 4 is incorrect because the goal of a pressure cooker is not to keep the food grains inside steam for a longer time. Rather, it uses the high pressure and elevated boiling point to cook the food quickly and efficiently.

Therefore, the correct answer is option 1.