The least number of 4 digits which is a perfect square is = ?

Answer: Option C

Least number of 4 digits is 1000
$$\begin{array}{rl}& 3|\overline{10}\overline{00}(31\\ & |9\\ & |------\\ & 61|100\\ & |61\\ & |------\\ & 39\\ & \therefore {\left(31\right)}^2<1000<{\left(32\right)}^2\\ & \text{Hence, required number}\\ & ={\left(32\right)}^2\\ & =1024\end{array}$$