Senescent phase of an organism's life span can be recognised by

Options

A. slow metabolism
B. cessation of reproduction
C. decreased immunity `
D. all of these

Explanation

### Step-by-Step Solution:

1. **Understanding the Phases of an Organism's Life Span**:
- An organism's life span can be divided into three main phases: the vegetative (or juvenile) phase, the reproductive phase, and the senescent phase.

2. **Identifying the Senescent Phase**:
- The senescent phase is the final stage of an organism's life span, characterized by aging and a gradual decline in physiological functions.

3. **Recognizing Characteristics of the Senescent Phase**:
- **Slow Metabolism**: During this phase, the metabolic rate of the organism decreases. This reduction in metabolic activity can lead to various physiological changes.
- **Cessation of Reproduction**: In this phase, organisms stop reproducing. For animals, this is often marked by the end of reproductive cycles (e.g., menopause in humans). In plants, this means they no longer produce flowers or seeds.
- **Decreased Immunity**: As organisms age, their immune system becomes less effective, leading to increased susceptibility to diseases.

4. **Conclusion**:
- All the characteristics mentioned (slow metabolism, cessation of reproduction, and decreased immunity) are indicative of the senescent phase. Therefore, the correct answer to the question is "all of these."

### Final Answer:
The senescent phase of an organism's life span can be recognized by **all of these**: slow metabolism, cessation of reproduction, and decreased immunity.

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