1. Identify the nature of glass

   Ordinary glass does not have a regular crystalline arrangement of particles. Its structure is amorphous.

2. Key property of amorphous solids

   Amorphous solids are often described as super-cooled liquids because:

   • they do not have a sharp melting point,
   • they soften gradually on heating,
   • their internal arrangement is disordered like that of a liquid.

3. Check the options

   • A: super-cooled liquid (\rightarrow) Correct. This is the standard textbook description of glass.
   • B: gel (\rightarrow) Incorrect. Glass is not classified as a gel.
   • C: polymeric mixture (\rightarrow) Incorrect. Though glass may contain network structures, it is not generally classified this way in this context.
   • D: micro-crystalline solid (\rightarrow) Incorrect. Glass is amorphous, not micro-crystalline.

4. Conclusion

   Therefore, glass is a super-cooled liquid.

$\boxed{\text{A}}$