Which one of the following groups of plants was domesticated in the 'New World' and introduced into the 'Old World'?

1. A

   Tobacco, cocoa and rubber

   (a) Tobacco, cocoa and rubber. All three are New World (Americas) domesticates later taken to the Old World: tobacco and cacao from the Americas, Hevea rubber from the Amazon. That trio is the official key.

2. B

   Tobacco, cotton and rubber

   (b) Tobacco, cotton and rubber. Cotton was independently old in the Old World (Gossypium arboreum in India/Africa) as well as in the Americas, so this mixed list is not the ‘New World introduced to Old World’ key.

3. C

   Cotton, coffee and sugarcane

   (c) Cotton, coffee and sugarcane. Coffee is Ethiopian/Old World; sugarcane is South-East Asian/Old World; cotton is not a one-way New World gift. This is not the key.

4. D

   Rubber, coffee and wheat

   (d) Rubber, coffee and wheat. Coffee and wheat are Old World; wheat is a Fertile Crescent crop. This is not the key.

Summary. Official key is (a) tobacco, cocoa and rubber. Columbian Exchange carried these American crops outward. Cotton, coffee, sugarcane and wheat have Old World homes, so lists that mix them fail the New-World-only test. Honour the stored letter (a).