Introduction of alien species into new area poses a threat to extinction of indigenous species due to
(a) their high nutrient requirement
(b) their symbiotic relationship
(c) absence of their natural predators
(d) more intraspecific competition.

Options

A. their high nutrient requirement
B. their symbiotic relationship
C. absence of their natural predators
D. more intraspecific competition.

Explanation

To solve the question regarding the introduction of alien species and their threat to indigenous species, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Alien Species**: Alien species are organisms that are introduced to a new environment where they do not naturally occur. These species can sometimes become invasive, meaning they spread rapidly and can disrupt local ecosystems.

2. **Analyzing the Options**:
- **Option (a)**: High nutrient requirement - While some alien species may have high nutrient requirements, this alone does not necessarily lead to the extinction of indigenous species.
- **Option (b)**: Symbiotic relationship - Alien species may form symbiotic relationships, but this does not directly threaten indigenous species unless it leads to competitive exclusion.
- **Option (c)**: Absence of their natural predators - This is a critical point. When alien species are introduced, they often lack natural predators in the new environment, allowing them to grow and spread unchecked. This can lead to them outcompeting indigenous species for resources.
- **Option (d)**: More intraspecific competition - This refers to competition within the same species. While it can occur, it does not directly relate to the threat posed to indigenous species.

3. **Identifying the Correct Answer**: Given the analysis, the most significant factor that allows alien species to threaten indigenous species is the absence of their natural predators. This allows them to proliferate and potentially outcompete or prey on native species, leading to their extinction.

4. **Conclusion**: Therefore, the correct answer is **(c) absence of their natural predators**.

### Final Answer:
(c) absence of their natural predators.