Detailed Explanation:

1. Size of the sun at dusk – Optical illusion: The sun appears larger near the horizon due to the moon illusion, a psychological effect where our brain misinterprets distance cues, though its actual angular size remains constant.

2. Colour of the sun at dawn – Optical illusion: The sun appears red/orange because its light travels through more atmosphere, causing Rayleigh scattering that removes blue light, creating a perceived color change.

3. Moon being visible at dawn – NOT an illusion: This is a real astronomical event where the moon's orbital position allows it to be visible during daylight hours.

4. Twinkle of stars – Optical illusion: Stars don't actually twinkle; their light is refracted by turbulent atmospheric layers, causing rapid brightness fluctuations.

5. Polestar being visible – NOT an illusion: The Pole Star (Polaris) is genuinely visible due to its alignment with Earth's rotational axis, making it appear stationary.

Answer: Option 3 (1, 2 and 4) are optical illusions.