The potatoes are called modified stem because of:

In potato the under ground portion is a modified stem in that it stores reserve food and has all characters of stem i.e. roots and shoots appear from it and it has buds (eyes) also. Potato is a tuber (that part carry out photosynthesis- the green color part seen in young potato). It is called a modified stem as it has scaly leaves, nodes, internodes and adventitious roots, characteristic to stems.

Hence answer is 'Both A and B'