Final answer:
Wheat is the most commonly grown crop in the world, surpassing the cultivation of other major crops like rice and corn.
Step-by-step explanation:
The most commonly grown crop in the world is wheat. Given the information provided, today corn is produced in most countries and ranks as the third-most planted field crop after wheat and rice. Therefore, the answer to the question is a) Wheat. It's essential to note that rice and corn are also leading crops with significant production figures, but wheat surpasses them in global cultivation.