Questions

It is Blackboard or Chalkboard which is Black when clean, and as you write on it with white chalk it becomes dirty in White color.

Terylene and nylon are synthetic fibres produced by a process of polymerisation. Terylene is a polyester and nylon, a poyamide.

Silk is purely a natural product obtained from silk worms.

Rayon is a man-made material which consists of purified cellulose in the form of long fibres. Cellulose occurs in the cell wall of plants. Rayon fibre is chemically identical to cotton but has a shine like silk. Since it is  basically derived from a natural source (cellulose) it is called a regenerated fibre.