**Core Concept**
The life span of a house fly is influenced by various factors such as availability of food, water, and suitable environmental conditions. House flies are known to have a relatively long life span compared to other insects, which makes them a significant public health concern.

**Why the Correct Answer is Right**
The correct answer, **C. 1 month**, is based on the fact that the life span of a house fly is approximately 20-30 days, with some females living up to 1 month. This is due to their ability to adapt to different environments and feed on a wide range of organic matter. The life cycle of a house fly consists of four stages: egg, larva, pupa, and adult. During this time, they undergo metamorphosis and develop into mature flies capable of reproduction.

**Why Each Wrong Option is Incorrect**
**Option A:** 5 days is an underestimation of the life span of a house fly, as they can live significantly longer than this period.

**Option B:** 10 days is still too short, considering the average life span of a house fly can range from 20-30 days.

**Option D:** 1 year is an overestimation, as the life span of a house fly is typically limited to a few weeks.

**Clinical Pearl / High-Yield Fact**
House flies are a significant public health concern as they can spread diseases such as dysentery, cholera, and typhoid fever through their feces and saliva. They can also contaminate food and water sources, leading to outbreaks of gastrointestinal illnesses.

**✓ Correct Answer: C. 1 month**