Which of the following mammals is an egg laying species, that is, a monotreme which lays eggs instead of giving birth to live young ones?

Introduction / Context:
While most mammals give birth to live young, a very small group known as monotremes reproduce by laying eggs. Questions about monotremes are common in biology and general knowledge exams because they highlight an unusual reproductive mode in mammals. This question asks which mammal among the given options lays eggs, testing recognition of a classic example of a monotreme.

Given Data / Assumptions:

• The animals listed are Bat, Whale, Weasel, Platypus, and an extra decoy.
• We need to identify the mammal that lays eggs.
• We assume knowledge that monotremes are egg laying mammals from Australia and nearby regions.
• We also assume that only one correct monotreme example appears in the options.

Concept / Approach:
Monotremes are primitive mammals that lay eggs but also produce milk to feed their young. The best known monotremes are the platypus and echidnas, primarily found in Australia and New Guinea. Standard biology references emphasise that the platypus is an egg laying mammal with a duck like bill and beaver like tail. Other mammals listed such as bats, whales, and weasels are placental mammals that give birth to live young and do not lay eggs. Thus, the correct approach is to identify the monotreme among the options, which is the platypus.

Step-by-Step Solution:
Step 1: Recall that monotremes are mammals that lay eggs instead of giving live birth. Step 2: Remember the classic examples of monotremes, especially the platypus and echidnas. Step 3: Check each option to see whether it matches a known monotreme. Bat, whale, and weasel are not monotremes. Step 4: Recognise that Platypus is directly listed as one of the options. Step 5: Conclude that Platypus is the only egg laying mammal in the list and hence the correct answer.

Verification / Alternative check:
For verification, recall key characteristics of the platypus. It is a semi aquatic mammal native to eastern Australia, with a duck like bill, webbed feet, and the ability to lay eggs in burrows. After the eggs hatch, the mother nurses the young with milk secreted from mammary gland areas. This combination of egg laying and milk production is a defining feature of monotremes. The other animals listed are all known to give birth to live young ones, confirming that they are not monotremes and reinforcing that platypus is the correct choice.

Why Other Options Are Wrong:
Option A: Bat is a placental mammal that gives birth to live young and is not an egg layer. Option B: Whale is a marine mammal that produces live offspring and nurses them with milk, not by laying eggs. Option C: Weasel is a small carnivorous mammal that gives birth to live young and is not a monotreme. Option E: Deer is a herbivorous placental mammal and gives birth to live fawns.

Common Pitfalls:
Some students may confuse monotremes with marsupials or think that unusual mammals like bats somehow lay eggs. Another common mistake is to forget that even aquatic mammals like whales still give birth to live young and are not exceptions. To avoid such errors, it is useful to memorise that only a few mammals, specifically the platypus and echidnas, are egg laying monotremes. Recognising these names instantly helps answer such questions correctly.

Final Answer:
The mammal among the options that lays eggs and is a monotreme is the platypus.