A sample of basalt has smaller crystals than a sample of granite. What is the most likely reason for this?

The most likely reason is that basalt is an extrusive rock, while granite is an intrusive rock. What this means is that basalt forms at the surface of the earth, where temperatures are cooler. Because of the cooler external temperature, basalt cools relatively quickly, and there isn't as much time for mineral crystals within the rock to align to form larger crystals while the rock is cooling. Granite, on the other hand, forms below the surface of the earth, where temperatures are significantly higher. Therefore, it doesn't cool as quickly, and the minerals within granite have more time to form larger crystals and the rock cools.