Lisa gave four English tests last week. The score of her fourth test

Great question, Bunuel!

Lisa gave four English tests last week. The score of her fourth test was 8 more than average score of her first three tests.
Let A, B, C and D represent the scores of the 4 tests (D is the fourth test)
From the given information, we can write: (4th score) - (average of first three tests) = 8
Substitute to get: D - (A + B + C)/3 = 8
Multiply both sides of the equation by 3 to get: 3D - (A + B + C) = 24

How much greater is her fourth test score than the average score of four tests?
In other words, we want to find the value of: (4th score) - (average all four tests)
Let's let x be the desired value.
In other words: (4th score) - (average all four tests) = x (and we want to find the value of x)
Substitute to get: D - (A + B + C + D)/4 = x
Multiply both sides of the equation by 4 to get: 4D - (A + B + C + D) = 4x
Simplify to get: 3D - (A + B + C) = 4x

Since we already learned that 3D - (A + B + C) = 24, we can substitute 24 for 3D - (A + B + C) to get 24 = 4x
Solve: x =6

Answer: C