Introduction:
In this question, we are asked to identify the crop that was not known to our ancestors until about five centuries ago. We need to choose the correct option from the given choices: potato, rice, wheat, and cotton. Let's analyze each option to determine the correct answer.

Analysis:
1. Potato: The potato is native to the Andes region in South America and was first domesticated by the indigenous people of this region around 7,000 to 10,000 years ago. It was introduced to Europe after the Spanish conquest of the Americas in the 16th century. Therefore, our ancestors did not know about potatoes until about five centuries ago. This makes option A, potato, the correct answer.

2. Rice: Rice cultivation has a long history, dating back to at least 5,000 BC in China. It spread to other parts of Asia and eventually reached Europe and other continents through trade and exploration. Therefore, rice was known to our ancestors for thousands of years and is not the correct answer.

3. Wheat: Wheat is one of the oldest cultivated crops in the world, with evidence of its cultivation dating back to around 10,000 BC in the Fertile Crescent of the Middle East. It played a crucial role in the development of human civilization and was known to our ancestors for thousands of years. Therefore, wheat is not the correct answer.

4. Cotton: Cotton cultivation can be traced back to ancient civilizations in India, Egypt, and China, with evidence of cotton fabrics dating back to around 3000 BC. It was known to our ancestors for several millennia and is not the correct answer.

Conclusion:
After analyzing each option, we can conclude that the crop that was not known to our ancestors until about five centuries ago is the potato (option A). The potato was introduced to Europe after the Spanish conquest of the Americas in the 16th century, and our ancestors did not know about it until then.