To find the correct equation representing the relationship between the cost of the cookies, \( y \), and the number of cookies bought, \( x \), during the sale, we first determine the price per cookie during the sale.
The original price for 4 cookies is $2.00, which means the price per cookie is:
\[ \text{Original price per cookie} = \frac{2.00}{4} = 0.50 \]
During the sale, each cookie is $0.10 less:
\[ \text{Sale price per cookie} = 0.50 - 0.10 = 0.40 \]
Now we can write the equation for the total cost \( y \) based on the number of cookies \( x \):
\[ y = 0.40x \]
Therefore, the correct response is:
y = 0.40x