The correct answer is: C. 60 : 35 : 5
Fly ash is a byproduct of coal-fired power plants. It is a fine, powdery material that is rich in silica and alumina. Sand is a granular material that is composed of silica and other minerals. Lime is a mineral that is composed of calcium oxide.
The mix of fly ash, sand, and lime for making fly-ash building bricks is typically 60 : 35 : 5. This mix provides the best balance of strength, durability, and workability.
Option A: 80 : 13 : 7 is too much fly ash and not enough sand and lime. This mix would result in bricks that are weak and not durable.
Option B: 70 : 20 : 10 is too much sand and not enough fly ash and lime. This mix would result in bricks that are weak and not durable.
Option D: None of these is the correct answer.