What interpretations can be made about the maturity of a volcano where most eruptions occur from basal fissures and there are a number of parasitic cones along the volcano's flanks?
- mature volcano
- early stages of growth

These are called as Lateral blasts.

Explanation:

The parasitic cone is a cone-shaped vent from where the reported lava comes out of the earth's crust. As the flank of the volcano is unstable. And eventually, the flank eruption leads to the formation of a parasitic cone. These mark a mature stage of growth of volcanoes. Lateral eruptions are typically formed near the rift zones where a volcano is breaking apart. An example can be given of Mt. St Helens, and Mayon volcano. The lateral explosion leads to the pyroclastic flows and lahars to occur and can span hundreds of square kilometers. Since these flanks are an easy to top break and molten lava thus escape out quickly.

I'm pretty sure it's c. extremely gradual but not 100% sure though.