Respiratory organs like gills, book gills, book lungs or tracheal system are found in phylum
(a) Arthropoda
(b) Aschelminthes
(c) Mollusca
(d) Chordata

Options

A. Arthropoda
B. Aschelminthes
C. Mollusca
D. Chordata

Answer

To solve the question regarding which phylum contains respiratory organs such as gills, book gills, book lungs, or a tracheal system, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Respiratory Organs**: The question lists several types of respiratory organs: gills, book gills, book lungs, and a tracheal system.

2. **Analyze Each Option**:
- **(a) Arthropoda**: This phylum includes various classes such as crustaceans, insects, and arachnids. Crustaceans possess gills, while arachnids (like spiders) have book lungs. Insects have a tracheal system for respiration.
- **(b) Aschelminthes**: This phylum, also known as nematodes or roundworms, does not typically have specialized respiratory organs like gills or lungs. They generally rely on diffusion for gas exchange.
- **(c) Mollusca**: While some mollusks have gills (like aquatic mollusks), they do not have book gills or a tracheal system.
- **(d) Chordata**: This phylum includes animals with a backbone. While many chordates have gills (like fish), they do not have book gills or a tracheal system.

3. **Determine the Correct Answer**: Since Arthropoda includes organisms that have all the mentioned respiratory organs (gills in crustaceans, book lungs in arachnids, and a tracheal system in insects), the correct answer is **(a) Arthropoda**.

### Final Answer:
**(a) Arthropoda**