Right answer:
D. una toalla
If a person wants to dry, then he or she must use a towel (toalla), which is a piece of material that you use for drying your hands or body, or for drying dishes. Since toalla is a feminine singular noun, we use the feminine singular definite article, which is una. Recall that the indefinite article is used to refer to a particular person or thing when the reader or listener doesn't know which one is being described.