In which earth layer does the pressure reach 3.5 million atmospheres?

The correct answer is - Inner core.

The inner core is the layer that has the highest pressure of all the layers of our planet, the Earth. The pressure in the inner core is enormous, unimaginable by our standards, as it varies from 3.3 million atmospheres to 3.6 million atmospheres.

In general, the pressure in the Earth's layers becomes bigger with the depth. So we can say that the surface has the smallest pressure, than the upper crust increases slightly with its pressure, the mantle has significantly higher pressure, and the outer core has a pressure much bigger than the mantle, with the central part of the Earth, the inner core, being the one with the highest pressure of all layers.