What is the least number of soldiers that can be drawn up in troops of 12, 15, 18 and 20 soldiers and also in form of a solid square?

Answer: Option A

In this type of question, We need to find out the LCM of the given numbers.
LCM of 12, 15, 18 and 20;
$$\begin{array}{rl}& 12=2\times 2\times 3;\\ & 15=3\times 5;\\ & 18=2\times 3\times 3;\\ & 20=2\times 2\times 5;\end{array}$$
Hence, LCM = $$2\times 2\times 3\times 5\times 3$$
Since, the soldiers are in the form of a solid square.
Hence, LCM must be a perfect square. To make the LCM a perfect square, We have to multiply it by 5,
hence,
The required number of soldiers
= $$2\times 2\times 3\times 3\times 5\times 5$$
= 900