Final answer:
The correct option is b.
Shield volcanoes have the most gently sloping sides, formed by low-viscosity basaltic lava. Volcanoes near subduction zones are typically closer to coastlines because they arise from subducting tectonic plates near the Earth's surface next to the continental crust.
Step-by-step explanation:
Of the three primary forms of subaerial volcanoes, shield volcanoes have the most gently sloping sides, due to the low viscosity of the basaltic lavas that form them. Shield volcanoes, such as those found in Hawaii, are broad and dome-shaped with gentle slopes, built by the eruption of low-viscosity lava that can flow over great distances.
Volcanoes associated with subduction zones are generally found closer to the coastline because the volcanism is a result of magma formation due to the partial melting of the asthenosphere above the subducting slab, which occurs relatively near the Earth's surface and often close to the edges of the subducting tectonic plate. Therefore, the closer proximity of these volcanoes to the coast is due to the mechanics of plate subduction beneath continental crust adjacent to ocean basins.