Explanation:

To answer this question, we need to understand the units of measurement and how they are related to each other in terms of distance.

Units of Measurement:
- Gram (g): A unit of mass used to measure the weight of objects.
- Liter (L): A unit of volume used to measure the capacity of liquids or gases.
- Meter (m): A unit of length used to measure distances.

Relationship between Units:
- Mass is a measure of the amount of matter in an object, and weight is the force exerted on an object due to gravity. The two terms are often used interchangeably, but they are not the same.
- Volume is the amount of space occupied by an object or substance.
- Length is the distance between two points.

Walking:
Walking is an activity that involves moving from one place to another on foot. The distance covered while walking is measured in meters.

Answer:
The correct answer is option 'C' - One meter.

Explanation:
- When we walk, we cover a certain distance. This distance is measured in meters.
- The question states that it takes 30 minutes to walk. It does not mention any specific distance.
- Therefore, the answer cannot be in terms of mass (gram), volume (liter), or weight (kilogram), as these units are not directly related to the distance covered while walking.
- The only unit that directly relates to distance is the meter.
- Hence, the correct answer is option 'C' - One meter, as it represents the distance covered while walking.

In conclusion, the answer to the question "It takes 30 minutes to walk __________" is "One meter" (option 'C').