Birds avoid eating the Monarch butterfly because:

The correct answer is Option (2) → its body contains a special chemical that makes it highly distasteful

Prey species have evolved defences against predators. Some insects and frogs are cryptically coloured, so predators cannot spot them easily. Others are poisonous and therefore avoided. The Monarch butterfly is in the second group: a special chemical in its body makes it highly distasteful to birds. It gets this chemical as a caterpillar, by feeding on a poisonous weed.

Option (1) it is cryptically coloured and cannot be detected easily — Incorrect. Cryptic colouring is a real defence of some insects and frogs, but it is not the Monarch’s. Birds avoid the Monarch because of its taste, not because they fail to see it.

Option (2) its body contains a special chemical that makes it highly distasteful — Correct. The special chemical present in its body makes the butterfly highly distasteful to its predator, the bird.

Option (3) it secretes a poison from special glands on its wings — Incorrect. The Monarch does not make the chemical in glands. It takes it in from a poisonous weed during its caterpillar stage.

Option (4) its wings bear thorn-like spines — Incorrect. Thorns are a defence of plants such as Acacia and cactus. The Monarch’s defence is chemical.

NCERT Extract:
Population Interactions – “… Prey species have evolved various defenses to lessen the impact of predation. Some species of insects and frogs are cryptically-coloured (camouflaged) to avoid being detected easily by the predator. Some are poisonous and therefore avoided by the predators. The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body. Interestingly, the butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.”