Hiking generally refers to the activity of walking in nature, often on trails or designated routes, and can involve various terrains. Based on your options:
a. A designated hiking route - Yes, this is typically part of hiking. b. Going with a group of people - Not necessarily; hiking can be done solo or in groups. c. Walking on different terrains - Yes, this is a fundamental aspect of hiking. d. Spending the night in a sleeping bag - Not required for hiking; this describes camping.
So, the best options that constitute hiking are a and c.