Final answer:
Root crops like carrots are formed from modified roots, tubers like potatoes are formed from stem modifications, and bacterial tubers arise from bacterial growth, corresponding to option 'a'.
Step-by-step explanation:
The formation of root crops, tubers, and bacterial tubers is distinct for each category. Root crops are formed from plant roots that have been modified for food storage, examples being carrots, turnips, and beets. Tubers are formed from a modification of the stem; a familiar example is the potato. In potato plants, the tuber is a below-ground branch shoot that does not grow upwards but swells at its tips to form the tuber structure, and it contains 'eyes' which are lateral buds that can give rise to new plants. Contrary to root crops and tubers, bacterial tubers are formed by bacterial growth, although these are much less common in conventional agricultural practices.
The correct answer to the question would be option 'a': Root crops are formed from plant roots, tubers from stem modifications, and bacterial tubers from bacterial growth.