The three defining characteristics of chordates are :

Options

A. segmentation/dorsal hollow nerve cord/gill arches or pharyngeal pouches
B. dorsal hollow nerve cord/notochord/bilateral symmetry
C. well-developed coelom/gill arches or pharyngeal pouches/notochord
D. gill arches or pharyngeal pouches/dorsal hollow nerve cord/notochord

Explanation

To answer the question about the three defining characteristics of chordates, we will follow these steps:

### Step 1: Identify the Key Characteristics
Chordates are defined by certain key characteristics that they possess at some stage of their development. The four main characteristics include:
1. Dorsal hollow nerve cord
2. Notochord
3. Pharyngeal gill slits (or pharyngeal pouches)
4. Post-anal tail

### Step 2: Analyze the Options
Now, let’s analyze the options provided in the question to determine which three characteristics are defining for chordates.

- **Option A**: Segmentation, dorsal hollow nerve cord, gill arches or pharyngeal pouches
- **Analysis**: Segmentation is not a defining characteristic of chordates. Therefore, this option is incorrect.

- **Option B**: Dorsal hollow nerve cord, notochord, bilateral symmetry
- **Analysis**: While the dorsal hollow nerve cord and notochord are correct, bilateral symmetry is not a defining characteristic exclusive to chordates. This option is also incorrect.

- **Option C**: Well-developed coelom, gill arches or pharyngeal pouches, notochord
- **Analysis**: Well-developed coelom is not unique to chordates, as it can be found in other animal groups. Thus, this option is incorrect.

- **Option D**: Gill arches or pharyngeal pouches, dorsal hollow nerve cord, notochord
- **Analysis**: This option correctly lists three defining characteristics of chordates. Therefore, this option is correct.

### Step 3: Conclusion
The three defining characteristics of chordates are:
1. Dorsal hollow nerve cord
2. Notochord
3. Pharyngeal gill slits (or pharyngeal pouches)

Thus, the correct answer is **Option D**.

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