To find the correct mirror image of the given figure when the mirror is placed at line MN, you need to reflect each element horizontally across the line. Here's the step-by-step reasoning:

1. Observe the given figure:

   

2. In the mirror image, the sequence of elements will be reversed horizontally. Letters and numbers will appear as they would if viewed in a mirror.
3. Consider each element of the sequence "Unt57LE":
   • "U" will look like reversed "U",
   • "n" remains "n",
   • "t" remains "t",
   • "5" appears as a mirrored "5",
   • "7" appears as mirrored "7",
   • "L" appears as mirrored "L",
   • "E" appears as mirrored "E".
4. The sequence should read "EL75tnU" from right to left when mirrored.

Now, compare this mirrored sequence to the given options:

1. The correct mirror image is:

   

Therefore, the correct answer is option 2, which accurately reflects the given figure when a mirror is placed at MN.