To find the total cost \( c \) that Vicky paid for the oranges, we need to use the equation based on the number of pounds of oranges she bought and the cost per pound.
Vicky bought 3.5 pounds of oranges, and each pound costs $3.90. The total cost can be calculated using the formula:
\[ c = \text{(number of pounds)} \times \text{(cost per pound)} \]
Substituting the values:
\[ c = 3.5 \times 3.90 \]
Calculating this gives us:
\[ c = 13.65 \]
The equation representing the total cost Vicky paid for the oranges is:
\[ c = 3.5 \times 3.90 \]
Now, looking at the responses you've provided, we can see that the correct representation would align with:
\[ 3.90 \times 3.5 = c \quad \text{where Vicky paid $13.65.} \]
So the final answer would be:
\( 3.90 \times 3.5 = c \); Vicky paid $13.65.