Answer:
The correct option is 3.
Explanation:
The amount Katie should pay is defined by the given expression.
.... (1)
Katie bought following item:
Number of bottles of milk = 6
Number of bags of candy = 7
Number of pizza = 1
cost of a pizza = $11
Let x be the cost of a bottle of milk and y be the cost of a bag of candy.
Then the total amount Katie should pay is
.... (2)
On comparing (1) and (2), we get

It means the cost of each bottle of milk is the same as the cost of each bag of candy.
Therefore the correct option is 3.