Ratios are in proportion if they are equal or equivalent. This can be verified by using the product of means and product of extremes.

In a: b :: c: d; 'b' and 'c' are means and 'a' and 'd' are extremes.

For 4 kg : 20 kg :: ₹ 50 : ₹ 250

Product of means = 20 $\times$ 50 = 1000
Product of extremes = 4 $\times$ 250 = 1000
So, this is in proportion.

For 2 dozens : 3 dozens :: ₹ 40 : ₹ 80

Product of means = 3 $\times$ 40 = 120
Product of extremes = 2 $\times$ 80 = 160

So, this is not in proportion.

For  8 liters : 15 liters :: ₹ 25 : ₹ 50
Product of means = 15 $\times$ 25 = 375
Product of extremes = 8 $\times$ 50 = 400

So, this is also not in in proportion.

For  2 km : 3 km :: 1 m : 2m
Product of means = 3 $\times$ 1 = 3
Product of extremes = 2 $\times$ 2 = 4

So, this is also not in in proportion.