Step 1: Understand horizontal communication.
  
Horizontal communication occurs between individuals or groups at the same level in an organization. It is often used to share information, solve problems, or make decisions.  
Step 2: Evaluate the options.
  
- Upward communication is from lower levels to higher levels of management.  
- Downward communication flows from higher to lower levels of management.  
- Parallel communication flows between colleagues or teams at the same level and is the correct answer.  
Step 3: Conclude.
  
In horizontal communication, suggestions typically flow at the same level among peers or colleagues.  
  
 
Final Answer:  
\[
\boxed{\text{At parallel}}
\]