Concept:  The question tests on application of Mixtures

Solution:

4 gallons of 90% glycerine contains 0.9 * 4 = 3.6 gallons of glycerine and 0.4 gallons of water.

Let us add x gallons of water to make it a 75% glycerine (25% water solution)

Therefore 3.6/0.4+x=75/25

3.6 = 3(0.4 + x)

3x = 2.4

x = 0.8

 Option E