Under unfavourable condition, the $Amoeba$ withdraws its pseudopodia and secretes a three-layered hard covering around itself. This phenomenon is termed as

The correct option is B Encystation
Encystation
During unfavorable conditions the $Amoeba$ develops a three-layered tough wall around itself called a cyst. This process is known as encystation.

Sporulation
The encysted $Amoeba$ divides by multiple fission to form pseudopodiospores which are released upon the return of favourable conditions and grow into new individuals. This phenomenon is known as sporulation.

Binary fission
Binary fission is the method of asexual reproduction which is also seen in $Amoeba$ under favourable conditions. In this method, the nucleus elongates and divides into two followed by division of cytoplasm, giving rise to two daughter cells. Example $Amoeba$.

Final answer
(b) Encystation