Here,
Volume of 1 small cube = $5^3 = 125$

Total volume of all 3 cubes = $3 * 125 = 375$

Now, let check with each option

a) 6.9 : Volume of the cube = $(6.9)^3 = 328$. This is not possible since the volume of larger cube aquarium < 3 small cube aquarium

b) 8.4 : Volume of the cube = $(8.4)^3 = 592$. Now 50% of 592 = 296 = This is one possible solution, since 375 (smaller cube aquarium) > 50% of Larger aquarium

c) 9.5 : Volume of the cube = $(9.5)^3 = 857$. Now 50% of 857 = 428 = This is not possible as the volume of smaller cubes = 375 and is < 50% volume of larger aquarium

Option B