To determine the total cost \( c \) that Vicky paid for the 3.5 pounds of oranges, we can use the equation that represents the relationship between the cost per pound and the total cost:
\[ c = \text{(cost per pound)} \times \text{(number of pounds)} \]
In this case:
\[ c = 3.90 \times 3.5 \]
Now, calculating the total cost:
\[ c = 3.90 \times 3.5 = 13.65 \]
So, the correct equation that represents the total cost is:
\[ c = 3.90 \times 3.5 \]
Thus, the appropriate response is:
\[ 3.90 \times 3.5 = c; \text{ Vicky paid } $13.65. \]