What feature makes the phase diagram of water unusual? chemistry multiple choice below?

The answer : "The negative slope of the solid-liquid equilibrium line."

Generally, in a phase diagram, the slope of the solid-liquid equilibrium line (fusion curve) is inclined in a positive slope. This shows that the solid has a smaller volume than the solid.

In the phase diagram of water, the slope of the solid-liquid equilibrium line is inclined in a negative slope. The shows that solid water (ice) has a larger volume than liquid water.