c) Radish and carrot
Homologous structures are those that are related to common ancestry. Such structures have a common structural plan and are adapted to suit different functional roles. Of the given options, potato is a storage stem, tomato is a fruit (in fact, it is a berry), radish and carrots are roots. Since both radish and carrots are roots, they have similar structural plans and are homologous.
Radish and carrot : Carrot and radish are homologous structures because they are modifications of root for food storage. They contain different sets of nutrients and have different functions.
Radish and potato : Radish are modification of roots while potato is modification of stem so they are not homologous but they both store food so serve as analogous organs.
Carrot and tomato : Tomato is not a kind of modification and is not underground.