The correct statement is (a): “conversion of solid into vapours without passing through the liquid state is called sublimation”.
Explanation: Sublimation is the direct transition of a solid to a gas without going through the liquid phase.
Why the other options are incorrect:
(b):
“Conversion of vapours into solid without passing through the liquid state is called freezing”: This is the definition of deposition, the reverse of sublimation.
(c):
“Conversion of vapours into solid without passing through the liquid state is called freezing”: This is also the definition of deposition.
(d):
“Conversion of solid into liquid is called sublimation”: This is incorrect, the conversion of solid into liquid is called melting.