The practical limit of moisture content achieved in air drying of timber is A. 5% B. 15% C. 25% D. 35%

The correct answer is: B. 15%

The practical limit of moisture content achieved in air drying of timber is 15%. This is because, at this moisture content, the timber is no longer susceptible to decay or fungal attack. Additionally, the timber will be dimensionally stable and will not warp or crack.

Option A (5%) is incorrect because timber with a moisture content of 5% is too dry and will be susceptible to cracking and splitting. Option C (25%) is incorrect because timber with a moisture content of 25% is still susceptible to decay and fungal attack. Option D (35%) is incorrect because timber with a moisture content of 35% is too wet and will not be dimensionally stable.