The correct option is D 24
Since the number of rows and columns of saplings have to be equal, the number of saplings should be a perfect square.
The gardener has 1000 saplings.
In order to estimate the square root of 1000, we need to find two consecutive perfect squares between which 1000 lies.
961<1000<1024
We can see that, 312=961 and 322=1024
961<1000<1024
31<√1000<32
Hence, required number of additional saplings=1024−1000=24
1000+24=1024
Hence, the gardener needs 24 more saplings.