[A]: 'Ontogeny recapitulates phylogeny' was the concept put forward by Haeckel.
[R]: The embryonic stages show the course of evolution which an adult has gone through during development

Options

A. (a) If both A and R are true and R is the correct explanation of A
B. (b) If both A and R are true but R is not the correct explanation of A
C. (c) If A is true and R is false
D. (d) If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states, "Ontogeny recapitulates phylogeny," which is a concept proposed by Ernst Haeckel. This means that the development of an individual organism (ontogeny) reflects the evolutionary history of that organism's species (phylogeny).

**Hint**: Remember that "ontogeny" refers to individual development, while "phylogeny" refers to evolutionary history.

2. **Explaining the Concept**: Haeckel's theory, known as the Biogenetic Law or the theory of recapitulation, suggests that during its embryonic development, an organism passes through stages that resemble the adult forms of its ancestors. This implies that the embryonic stages of an organism reflect the evolutionary stages of its species.

**Hint**: Think of embryonic development as a timeline that shows the evolutionary journey of a species.

3. **Analyzing the Reason**: The reason states that "the embryonic stages show the course of evolution which an adult has gone through during development." This aligns perfectly with the assertion, as it emphasizes that the embryonic development reflects the evolutionary history of the adult organism.

**Hint**: Focus on how the embryonic stages mirror the evolutionary changes that have occurred over time.

4. **Conclusion**: Both the assertion and the reason are true. The reason provided is a correct explanation of the assertion. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

**Hint**: When evaluating assertion-reason questions, check if the reason logically supports the assertion.

### Final Answer:
Both assertion and reason are true, and the reason is the correct explanation of the assertion.