$(a)$ Which major invention led to the discovery of the microscopic world?
$(b)$ Name a single cell which may constitute a whole organism. What are they called?
$(c)$ Every multicellular organism has come from a single cell. Justify the statement.

(N/A) The invention of the microscope by $Antony$ $van$ $Leeuwenhoek$ led to the discovery of the microscopic world.
$(b)$ $Amoeba$ is a single cell that constitutes a whole organism. Such organisms are called unicellular organisms.
$(c)$ Every multicellular organism begins its life as a single cell called the zygote. This zygote is formed by the fusion of male and female gametes during fertilisation. Through repeated cell divisions,this single cell develops into a complex multicellular organism.