Which of the following is used as a substitute of glass?

The correct answer is option 3. PMMA.

Among the options provided, PMMA (Polymethyl methacrylate) is commonly used as a substitute for glass.

PMMA, also known as acrylic or acrylic glass, is a transparent thermoplastic material that has optical properties similar to glass but is lighter, more impact-resistant, and less prone to shattering. It is often used as a substitute for glass in various applications, including windows, skylights, signage, aquariums, and lenses for eyeglasses.

Glyptal and urea formaldehyde resin are not typically used as substitutes for glass. Glyptal is a type of alkyd resin used primarily as an insulating coating or adhesive, while urea formaldehyde resin is used in wood products, adhesives, and molded objects but not as a direct substitute for glass.

So, the correct answer is PMMA.