A stain removal product is applied to a fabric. The graph shows the percentage of stain left on the fabric over time. How long does it take for 50% of the stain to be removed?

The question asks how long it takes to remove 50% of the stain. If 50% is removed, then $100\% - 50\% = 50\%$ of the stain is **left**.

The time corresponding to 50% stain left is approximately 2.5 minutes.

The correct option is B.
$$
\boxed{\text{B. 2.5 minutes}}
$$

At 4 minutes, approximately 30% of the stain is **left**.

Percentage removed = $100\% - \text{Percentage left}$  
Percentage removed = $100\% - 30\% = 70\%$.

*Note: The question asks for the percentage of stain removed, which is 70%. However, 70% is not among the given options. Option B is 30%, which is the percentage of stain **left**. Assuming there might be a slight misphrasing in the question and that it intends to ask for the percentage of stain left, we select 30%.*

The correct option is B (interpreting as percentage left).
$$
\boxed{\text{B. 30\%}}
$$

At 10 minutes, approximately 20% of the stain is **left**.

The correct option is D.
$$
\boxed{\text{D. 20\%}}
$$