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To answer these questions, we will analyze the graph of marginal utility (MUx) as a function of the quantity consumed of commodity X.

• The y-axis (vertical) represents marginal utility (MUx).
• The x-axis (horizontal) represents the quantity of X consumed.
• The marginal utility curve is downward-sloping.
• Marginal utility is zero (MUx = 0) when the quantity of X is 6 units.
• Marginal utility is negative when the quantity of X is greater than 6 units.

a) At what level(s) of consumption of commodity X is:

• i) the consumer at the point of saturation; The point of saturation is reached when the marginal utility (MUx) is equal to zero. From the graph, this occurs when the quantity of X consumed is 6 units.

  $$\boxed{\text{6 units}}$$

• ii) the marginal utility of X (MUx) decreasing but positive; Marginal utility is decreasing throughout the graph. It is positive when it is above the x-axis (MUx > 0). This occurs for quantities of 1, 2, 3, 4, and 5 units.

  $$\boxed{\text{1, 2, 3, 4, 5 units}}$$

• iii) the total utility of X (TUx) decreasing; Total utility (TUx) begins to decrease when marginal utility (MUx) becomes negative. From the graph, MUx is negative for quantities greater than 6 units, i.e., from 7 units onwards.

  $$\boxed{\text{7 units and above}}$$

• iv) the total utility of X (TUx) at its maximum? Total utility (TUx) is at its maximum when marginal utility (MUx) is equal to zero. From the graph, this occurs when the quantity of X consumed is 6 units.

  $$\boxed{\text{6 units}}$$