Which of the following statements is/are incorrect ?
(i) The liver fluke, a parasite, depends on intermediate hosts (a snail) to complete its life cycle.
(ii) The malarial parasite needs a vector (mosquito) to spread to other host organisms.
(iii) In case of brood parasitism, the eggs of parasitic birds are not detected and removed from the nest because the parasite's eggs resemble the host's eggs in morphology and colour.
(iv) A population of frogs protected from all predators would increase indefinitely.

Options

A. (i) and (iv)
B. (iii) and (iv)
C. Only (i)
D. None of these

Answer

To determine which of the statements provided in the question are incorrect, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement (i)**: "The liver fluke, a parasite, depends on intermediate hosts (a snail) to complete its life cycle."
- **Analysis**: This statement is correct. The liver fluke (Fasciola hepatica) indeed requires a snail as an intermediate host to develop its larval form before it can infect the definitive host, which is usually a mammal like cattle or sheep.

2. **Statement (ii)**: "The malarial parasite needs a vector (mosquito) to spread to other host organisms."
- **Analysis**: This statement is also correct. The malarial parasite, Plasmodium, is transmitted to humans through the bite of infected female Anopheles mosquitoes, which act as vectors for the disease.

3. **Statement (iii)**: "In case of brood parasitism, the eggs of parasitic birds are not detected and removed from the nest because the parasite's eggs resemble the host's eggs in morphology and colour."
- **Analysis**: This statement is correct. In brood parasitism, the eggs of the parasitic bird often mimic the appearance of the host's eggs, which helps them avoid detection and removal by the host.

4. **Statement (iv)**: "A population of frogs protected from all predators would increase indefinitely."
- **Analysis**: This statement is correct as well. Without predators, the population of frogs would not have any natural checks on their growth, leading to potential exponential population increase.

### Conclusion:
After analyzing all four statements, we find that all of them are correct. Therefore, the answer to the question "Which of the following statements is/are incorrect?" is that none of the statements are incorrect.

### Final Answer:
**None of the statements are incorrect.**