**Core Concept**
The question is testing the student's knowledge of enteric parasites and their characteristics. The key concept here is the size of the ova, which helps in identifying the causative organism.

**Why the Correct Answer is Right**
The ova size of 100 micrometres points towards the causative organism being *Toxocara* species. Toxocara ova are typically around 100 micrometres in size and are known to cause visceral larva migrans in children. The other options have different ova sizes:
- *Ascaris lumbricoides* ova are around 70-80 micrometres in size.
- *Enterobius vermicularis* ova are around 50-60 micrometres in size.
- *Trichuris trichiura* ova are around 50-55 micrometres in size.

**Why Each Wrong Option is Incorrect**
**Option A:** *Ascaris lumbricoides* ova are smaller than 100 micrometres, making it an unlikely cause.
**Option B:** *Enterobius vermicularis* ova are also smaller than 100 micrometres, ruling it out as a possible cause.
**Option D:** *Trichuris trichiura* ova are also smaller than 100 micrometres, making it an incorrect choice.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the size of the ova can help in identifying the causative organism. For example, *Toxocara* ova are typically larger than 100 micrometres, while *Ascaris lumbricoides*, *Enterobius vermicularis*, and *Trichuris trichiura* ova are smaller.

**Correct Answer: C.**