Final answer:
The invention of the astrolabe and compass by the Arabs and Chinese made European exploration possible.
Step-by-step explanation:
The correct answer is (a) astrolabe, compass.
In the fifteenth and sixteenth centuries, European exploration was made possible by the invention of the astrolabe and compass by the Arabs and Chinese. The astrolabe, a device that used constellations as a guide, helped mariners find their north-south position on the earth's surface. The compass, on the other hand, provided directional guidance by indicating the magnetic north. These inventions allowed European sailors to navigate confidently and venture into the open ocean, beyond sight of land.