(c) Radish and carrot
Radish and carrot, since they are similar in basic fundamental structure and origin, but dissimilar in function. Homologous structures are similar in origin but perform different functions. Both carrots as well as radish are underground roots. So they belong to the homologous category.
a. potato is stem and carrot is root
b. potato is stem and radish is root
c. tomato is fruit and carrot is root
So, they differ in structure and nature and hence are not homologous organs.