Final answer:
Earthquakes within a continental plate can occur due to the b) stretching and thinning of the continental crust or the d) collision of two continental plates. Subduction of continental plates is very rare, and the transformation of basaltic magma into granite is unrelated to earthquake generation.
Step-by-step explanation:
Earthquakes within a continental plate can be generated by several mechanisms related to the movement and interaction of tectonic plates. Earthquakes are closely associated with the boundaries of these plates and their different types of interactions.
Option (a), which suggests that earthquakes may be caused by the subduction of one continental plate beneath another, is highly unlikely and pretty rare because the thick continental masses cannot readily subduct due to their buoyancy.
Option (b) is a potential cause, as the stretching and thinning of the continental crust can lead to faulting and seismic activity, a process often seen in rift zones. Option (c) is incorrect because the transformation of basaltic magma into granite does not directly cause earthquakes. Lastly, option (d) is also a valid cause, since the collision of two continental plates can create earthquakes alongside the creation of mountain ranges, such as the impact that formed the Alps.
In conclusion, the correct methods in which earthquakes may be generated within a continental plate are options (b) stretching and thinning of the continental crust, and (d) collision of two continental plates. These processes are critical in understanding the dynamic nature of Earth's geology and the formation of various geological features.