In what part of a volcano would you expect to find rocks with the coarsest grains

The correct answer is: in the magma chamber.

Explanation:

The volcanoes are creating igneous rocks, and they can be intrusive and extrusive. The intrusive ones are forming deep into the ground, while the extrusive are forming on the surface. Where the rocks are formed makes a big difference in their crystals size. The rocks that form by magma that has cooled the deepest into the ground would have the largest crystals, thus the coarsest grains. The reason for this is that the magma that has been cooling the deepest into the ground will be exposed to higher temperatures and pressures, which will lead to a very slow cooling off. Because the cooling off will be at such a slow rate, the crystals will have enough time to became much larger. The deepest part of a volcano is its magma chamber, thus it will be the place that will produce the coarsest grained igneous rocks.

see earlier answer