To determine how much 7 pounds of grapefruit will cost, we first need to find the cost per pound of grapefruit. Mr. Yang purchased 5 pounds for $4.90.
First, we calculate the cost per pound:
\[ \text{Cost per pound} = \frac{\text{Total cost}}{\text{Total pounds}} = \frac{4.90}{5} = 0.98 \text{ dollars per pound} \]
Next, we find the cost for 7 pounds:
\[ \text{Cost for 7 pounds} = \text{Cost per pound} \times \text{Number of pounds} = 0.98 \times 7 \]
Calculating that:
\[ 0.98 \times 7 = 6.86 \]
Therefore, the cost for 7 pounds of grapefruit is $6.86.
So the correct answer is C. $6.86.