Final answer:
Helium will not freeze even at absolute zero unless a pressure of 2.5 MPa is applied. It is unique in this regard as it requires both a very low temperature and high pressure to become a solid due to its weak intermolecular forces.
Step-by-step explanation:
The substance that will not freeze even at absolute zero unless a pressure of 2.5 MPa is applied is Helium (a). Unlike substances like water, oxygen, and carbon dioxide, helium remains a gas at very low temperatures due to its very weak intermolecular forces. It requires both an extremely low temperature and the application of a high pressure to transition to a solid state.
At pressures below that of the triple point, a substance can exist as either gas or solid through a process called sublimation. Water, for example, will not have a liquid phase at pressures below 0.00600 atm, and carbon dioxide sublimates at a standard atmospheric pressure of 1 atm.
Helium is the only element that cannot be solidified by lowering the temperature alone; it requires the additional factor of applying a pressure above 2.5 MPa, which is a very unique characteristic among the elements. Its property of remaining a gas until extremely high pressures are applied makes it exceptional in the realm of elements.