Final answer:
To distinguish between a root hair and a very small root, you should look for the presence of a vascular core. Root hairs don't have vascular tissue, while even tiny roots will have a vascular system and may develop lateral roots.
Step-by-step explanation:
You can distinguish between a root hair and a very small root by looking for the presence of vascular tissue. Root hairs are single cells that extend from the epidermis of a root, and they do not contain vascular tissue. In contrast, even very small roots will have a vascular core or stele, which is essential for the transport of water and nutrients. Additionally, unlike root hairs, very small roots can give rise to lateral roots, since they contain a pericycle from which these can develop.
Moreover, roots have a protective structure called a root cap at their tips, which root hairs do not possess. The correct answer to the question is therefore the presence of vascular tissue (b), as it is the key difference between a root hair and an actual root.