Hiking generally refers to the activity of walking over natural terrains, often on designated trails or paths. Based on your options:
A. Designated hiking route - This is a common aspect of hiking, but not a strict requirement.
B. Going with a group of people - While hiking can be done in groups, it is not a defining characteristic.
C. Walking on different terrains - This is a key component of hiking, as it involves traversing various landscapes.
D. Spending the night in a sleeping bag - This describes backpacking or camping, which can be part of a hiking experience but is not essential for hiking itself.
In summary, C (Walking on different terrains) best captures the essence of hiking as a standalone activity.