A slower ascent rate primarily causes:

The correct choice identifies that a slower ascent rate primarily causes more on-gassing into the slow tissue compartments. During a dive, as a diver goes deeper, the body absorbs nitrogen from the breathing gas under pressure. When ascending, especially at a controlled, slower rate, the reduction in pressure allows for the safe release of this absorbed nitrogen.

In the slower tissue compartments—those tissues that take longer to off-gas nitrogen—this gradual ascent allows for more nitrogen to be released back into the bloodstream before it forms bubbles, which can lead to decompression sickness. If the ascent is too quick, the nitrogen can come out of solution too rapidly, forming bubbles that can cause harm.

The other options touch upon different aspects of diving physiology but do not directly address the primary effect of a slower ascent rate on nitrogen dynamics in the body. For example, less nitrogen absorption refers to the initial condition of nitrogen entering the body, not its release during ascent. Similarly, nitrogen narcosis is a condition caused primarily by increasing pressure, not the rate of ascent, and faster recovery is more associated with appropriate ascent rates rather than slower ones that may lead to complications.