(a) Sand stone
(b) Granite
(c) Marble
(d) Basalt
Ans :- (a) Sand stone
Explanation :-
Sand stone has less unit weight among the following :
| Type of rock | Unit weight |
| Sand stone | 22000 (N/m3) |
| Shale | 23000 (N/m3) |
| Lime stone | 25000 (N/m3) |
| Basalt | 26000 (N/m3) |
