The extremes in the proportion 3/4=15/20 are

3 and 4

3 and 20

15 and 4

15 and 20

A proportion is an equation where two ratios are equal and a proportion has given names and these are the extremes and the means. The extreme and the mean allows you to do cross multiplying. In the given proportion above 3/4 is the same with 3 : 4 and 15/20 is 15 : 20. The extremes are 3 and 20, whereas the means are 4 and 15. Therefore the correct answer would be the second option.