Aluminium has a specific gravity of

1.5
2.7
4.2
7.8

The correct answer is B. 2.7.

Specific gravity is the ratio of the density of a substance to the density of a standard substance. The standard substance is usually water, which has a density of 1 gram per cubic centimeter.

Aluminium has a density of 2.7 grams per cubic centimeter. This means that aluminium is 2.7 times as dense as water.

Option A is incorrect because 1.5 is not the specific gravity of aluminium.

Option C is incorrect because 4.2 is not the specific gravity of aluminium.

Option D is incorrect because 7.8 is not the specific gravity of aluminium.