Explanation:
Seasoning of timber is a process of removing moisture content from the timber to make it suitable for various construction purposes. The process of seasoning helps in improving the quality of timber and increasing its strength. The following points explain why seasoning of timber is done:

Decreasing moisture content:
The main purpose of seasoning timber is to reduce the moisture content of the wood. The process of seasoning involves drying the timber gradually under controlled conditions, which helps to remove the excess moisture from the wood. This process makes the timber more stable, durable and less prone to cracking, warping or splitting.

Improving quality:
Seasoning also helps to improve the quality of timber by reducing its weight, increasing its strength and hardness, and making it more resistant to decay and insect attack. The dried timber is less likely to shrink or expand, which makes it more suitable for construction purposes.

Enhancing workability:
Seasoned timber is easier to work with and can be cut, sawn, planed or drilled more easily than unseasoned timber. This is because the dried timber is harder and more compact, which makes it less prone to splintering or tearing during the machining process.

Preventing damage:
Unseasoned timber is more susceptible to damage from insects, fungi, and other pests. Seasoning helps to reduce the moisture content of the timber, making it less attractive to pests and reducing the risk of damage.

Therefore, the correct answer is option 'B' i.e. decreasing moisture content.