Craspedote (with velum) medusa is present in the pair
(a) Aurelia and Rhizostoma
(b) Pennatula and Aurelia
(c) Rhizostoma and Corallium
(d) Physalia and Halistemma

Options

A. Aurelia and Rhizostoma
B. Pennatula and Aurelia
C. Rhizostoma and Corallium
D. Physalia and Halistemma.

Answer

To determine which pair contains Craspedote (with velum) medusa, we need to analyze the options provided and identify the organisms present in each pair.

### Step-by-Step Solution:

1. **Understand the Term "Craspedote Medusa":**
- Craspedote medusa refers to a type of jellyfish that possesses a velum, which is a membrane that helps in swimming. This type is typically found in certain groups of jellyfish.

2. **Identify the Organisms in Each Option:**
- (a) **Aurelia and Rhizostoma**: Both are types of Scyphozoans, which are known to have craspedote medusa.
- (b) **Pennatula and Aurelia**: Pennatula is a type of soft coral and does not have a craspedote medusa, while Aurelia does.
- (c) **Rhizostoma and Corallium**: Rhizostoma is a Scyphozoan with craspedote medusa, but Corallium is a type of coral and does not have this feature.
- (d) **Physalia and Halistemma**: Physalia (Portuguese man o' war) is not a true jellyfish and does not have craspedote medusa, and Halistemma is also not known for this type.

3. **Evaluate Each Option:**
- **Option (a)**: Both Aurelia and Rhizostoma are Scyphozoans and exhibit craspedote medusa. This option is valid.
- **Option (b)**: Pennatula does not have craspedote medusa. This option is invalid.
- **Option (c)**: Corallium does not have craspedote medusa. This option is invalid.
- **Option (d)**: Neither Physalia nor Halistemma has craspedote medusa. This option is invalid.

4. **Conclusion:**
- The only pair that contains organisms with craspedote medusa is **Option (a) Aurelia and Rhizostoma**.

### Final Answer:
**(a) Aurelia and Rhizostoma**