Morphology is the study of

Options

A. a) Externally visible features
B. b) External appearance of the organ
C. c) Body wall
D. d) Both 1 and 2

Explanation

**Step-by-Step Solution:**

1. **Understanding Morphology**: Begin by defining the term "morphology." Morphology is a branch of biology that studies the form and structure of organisms. It encompasses various aspects, including the shape, size, and arrangement of different parts of an organism.

2. **Analyzing the Options**:
- **Option A**: Externally visible structure - This refers to the parts of an organism that can be seen without dissection or advanced imaging techniques. This is indeed a part of morphology.
- **Option B**: External appearance of the organ - This option also aligns with the definition of morphology, as it deals with how organs appear from the outside.
- **Option C**: Body wall - While this may be a structural component, it is not necessarily externally visible in all organisms, making it less relevant to the definition of morphology.
- **Option D**: Both 1 and 2 - This option suggests that both externally visible structures and external appearances of organs are included in the study of morphology.

3. **Determining the Correct Answer**: Since both options A and B accurately describe aspects of morphology, option D (both 1 and 2) is the most comprehensive and correct answer.

4. **Conclusion**: The correct answer to the question "Morphology is the study of" is option D, which includes both externally visible structures and the external appearance of organs.