Retrograde metamorphism a. happens under conditions of increasing temperature and pressure. b. may happen as regression brings rock closer to the Earth's surface. c. requires very high pressures but needs only low temperatures to occur. d. requires the addition of hydrothermal fluids to replace water driven off during prograde metamorphism.

b. may happen as regression brings rock closer to the Earth's surface.  

Explanation:

The term metamorphism when applied to geology refers to the changes that occur between rocks that allow them to form into different rocks over a cycle. Within this process, there is a type of metamorphism known as retrograde metamorphism, which occurs when there is a regression that brings rocks close to the surface and promotes a decrease in temperature and pressure.

it's actually d for edge-nuity.