12) C

We know that only 4 out of these 5 tiles are to be used to make the floor. 
For now let us add up all the number of black, white and grey squares on all 5 tiles. 

Number of black tiles
A = 2
B = 1
C = 3
D = 4
E = 5
Total = 15

Number of white tiles
A = 3
B = 6
C = 4
D = 3
E = 0
Total = 16

Number of grey tiles
A = 4
B = 2
C = 2
D = 2
E = 4
Total = 14

We need to remove a tile which can satisfy this equation:
Black tiles = white tiels = grey tiles

If we remove A, we get:
Black tiles = 15 – 2 = 13
White tiles = 16 – 3 = 14
Grey tiles = 14 – 4 = 10
So A is wrong

If we remove B, we get:
Black tiles = 15 – 1 = 14
White tiles = 16 – 6 = 10
Grey tiles = 14 – 2 = 12
So B is wrong

If we remove C, we get:
Black tiles = 15 – 3 = 12
White tiles = 16 – 4 = 12
Grey tiles = 14 – 2 = 12
So C is correct

If we remove D, we get:
Black tiles = 15 – 4 = 11
White tiles = 16 – 3 = 13
Grey tiles = 14 – 2 = 12
So D is wrong

If we remove E, we get:
Black tiles = 15 – 5 = 10
White tiles = 16 – 0 = 16
Grey tiles = 14 – 4 = 10
So E is wrong

C is the answer.

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