(a) Oceanic crust
(b) Continental crust
(c) The mantle
(d) The core
Ans :- (b) Continental crust

Explanation :-
The least dense rocks are found in continental crust.

• The average density of continental crust is about 2.83g/cm3, less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3.
• Continental crust is also less dense than oceanic crust, which density is about 2.6 g/cm3.