To find the total cost \( C \) that Vicky paid for the oranges, we can use the following formula:
\[ C = \text{(number of pounds)} \times \text{(cost per pound)} \]
In this case, Vicky bought 3.5 pounds of oranges, each costing $3.90. Thus, the equation to represent the total cost would be:
\[ C = 3.5 \times 3.90 \]
Calculating that:
\[ C = 3.5 \times 3.90 = 13.65 \]
So, the correct way to express the total cost and the number of pounds of oranges is:
\[ C = 3.5 \times 3.90 \]
Now, looking at the options given:
A. \( C/3.5 = 3.90 \) (This means \( C = 3.90 \times 3.5 \), which is incorrect)
B. \( 3.90C = 3.5 \) (This is incorrect because the equation doesn't correctly represent the total cost and price per pound)
C. \( C/3.90 = 3.5 \) (This means \( C = 3.90 \times 3.5 \), which is correct)
D. \( 3.90/3.5 = C \) (This is incorrect since it doesn't represent the total cost)
Thus, option C, \( C/3.90 = 3.5 \), is the one that correctly represents the total cost Vicky paid for the oranges, and she spent $13.65.