we have 1 litre = 1000cm³ (thanks Bunuel)
=>12 litres = 12*1000 = 12000cm³

Let call the height of the water is x
the breadth and length of the water will equal the aquarium
The volume of the water:
50 * 30 * x = 12000cm³
=> x = 12000/(50*30) = 8 cm

Answer is B

Another solution:

Let call the height of the space that doesn't contain water is x

First we must find the volume of the aquarium:
50 * 30 * 40 = 60000 cm³
The volume of the space that doesn't contain water :
60000-12000 = 48000 cm³
The height of the space:
50 * 30 * x = 48000 cm³
=> x = 48000/(50*30) = 32
The height of the water:
the height of the aquarium - x = 40 -32 = 8cm

Answer is B

Another approach
V1/V2=h1/h2

V1 & h1 are volume and height of container
V2 & h2 are volume and height of water to be filled
60000/12000=40/h2
h2=40/5=8cm
Ans B

Hi to get the height we have to convert liters into cm^3 .
As we know volume is measure in either liters or cm^3 or m^3
So to have LHS=RHS we should have variables in same quantity.

Conversion of liters to cum^3 is
1 Liters = 1000 Cm^3

Hope it helps