Let us discuss why A, B and D are wrong.
The easiest way to solve this is by calculating the tax for £800,000 (upper limit on the x-axis).
The tax on a house costing £800,000 = 4%
4% of £800,000 = £32,000
The graphs of A, B and D do not look like they are going to reach £32,000 for £800,000. Only C looks like it can do so. So C is the answer.