Rice types A, B, and C cost 30, 40, and 45 rupees per kg. In what ratio should a shopkeeper mix A:B:C to get a profit of 2.5 rupees when selling one kg of mixture at the price of type B?

 The selling price is the same as the price of type B, which is 40 rupees per kg. 
The profit on 1 kg is given to be 2.5 rupees, giving the cost price of 1 kg of the mixture to be 37.5 rupees. 

We need to mix 30, 40 and 45 in a ratio of 37.5 rupees per kg. 

We can eliminate option A, as that would be simply $$\frac{30+40+45}{3}=\frac{115}{3}=38.33$$ rupees per kg. 
We need to look for what option could lead to a price of 37.5 rupees per kg. 

Option B, on mixing A and B, would give a mixture of 35 rupees per kg, which, when combined with 2 units of type C, gives a mixture worth 40 rupees per kg (2 units of 35 and 2 units of 45)

Option C gives the correct answer. Mixing two units of type A and 1 unit of type C would give three units of mixture worth 35 rupees per kg, using alligation. These 3 units, when mixed with 3 units of type B, would give an average of 35 and 40, which is 37.5 rupees per kg. 

Option D on mixing B and C will give a mixture worth 42.5 rupees per kg, which, when combined with two units of type A, gives a final mixture worth 36.25 rupees per kg. 

Option E  on mixing A and B will give a mixture worth 35 rupees per kg, which, when mixed with 5 unit of type 45, this will be more than the mid-point of 40, and hence can be eliminated. 

Therefore, Option C is the correct answer.