The correct answer is: B. Sap from timber.
Seasoning is the process of drying timber to a moisture content that is suitable for its intended use. This process removes the sap from the timber, which can cause problems such as decay, splitting, and warping.
Knots are natural growths in the wood that can cause problems such as weakness and splitting. They are not removed by seasoning.
Twisted fibres are also a natural occurrence in wood and are not removed by seasoning.
Roughness of timber can be reduced by sanding or planing, but this is not a necessary part of the seasoning process.
In conclusion, seasoning of timber is essential to remove sap from the timber. This process helps to prevent problems such as decay, splitting, and warping.