Explanation:

- Mean proportional between 1.8 and 3.2 is v(1.8 × 3.2) = v5.76 = 2.4

- Third proportional to 5 and 3 is x such that 5 : 3 = 3 : x, so x = 3² / 5 = 9/5 = 1.8

- Ratio of mean proportional (2.4) to third proportional (1.8) is 2.4 : 1.8 = 4 : 3

- Option 1 (3 : 5) is not correct, as it doesn't match 4 : 3

- Option 2 (4 : 3) is the correct answer

- Option 3 (3 : 4) inverts the ratio

- Option 4 (5 : 3) does not relate to the values found