The value of photosynthetically active radiation above which no further increase in photosynthesis occurs is referred to as:

The right answer is the light saturation point.

More light usually results in higher photosynthesis levels. However, with the increase of the intensity of the light, the photosynthesis rate eventually reaches a maximum point. This point is called "luminous saturation point".

When this point is reached, the intensity of the light does not increase the rate of photosynthesis and the rate of photosynthesis curve ceases to increase.

Giga is the second highest prefix on the normal scale, the first is terra.

This is the most closely resembles the model.