Explanation:

To solve this, we use the concept of consumer equilibrium where the ratio of the marginal utility to price of each good should be equal.

- MU_X of X is 40.

- MU_Y of Y is 30.

- Price of Y is ?9.

- Consumer Equilibrium requires: \( \frac{MU_{X}}{P_{X}} = \frac{MU_{Y}}{P_{Y}} \).

Plug in the values for Y:

- \(\frac{30}{9} = \frac{MU_{X}}{P_{X}}\)

- \(\frac{10}{3} = \frac{40}{P_{X}}\)

Solve for \(P_{X}\):

- \(P_{X} = \frac{40 \cdot 3}{10} = 12\).

- Option 1: ?9 - This would imply equality if X also has a price that corresponds to a MU of 27, not 30.

- Option 2: ?30 - Implies lower utility per unit cost compared to Y.

- Option 3: ?15 - Implies mismatch in MU per price equivalency.

- Option 4: ?12 - Correct equilibrium price for X when matched with the given MU values and costs.

Correct Answer: Option 4: ?12