#[C#][1], 180 bytes

using System;public class P{public static void Main(string[]a){var d=(int)Math.Sqrt(a[0].Length);string f="";for(int i=0;i<d;i++){f+=a[0].Substring(i*d,d)+"\n";}Console.Write(f);}}

[Try Online][2] [1]: https://visualstudio.microsoft.com/ [2]: https://dotnetfiddle.net/r0RupR

C# , 180 bytes

using System;public class P{public static void Main(string[]a){var d=(int)Math.Sqrt(a[0].Length);string f="";for(int i=0;i<d;i++){f+=a[0].Substring(i*d,d)+"\n";}Console.Write(f);}}

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