Step 1: Understanding horizontal communication. 
  
In horizontal communication, suggestions, instructions, or information flow between people at the same hierarchical level in the organization. These communications typically occur at the same level and do not involve any authority structure. 
Step 2: Analyzing the options. 
  
(A) Upward: Incorrect. Upward communication refers to communication moving from subordinates to superiors. 
  
(B) Downward: Incorrect. Downward communication refers to communication from superiors to subordinates. 
  
(C) At parallel: Correct. In horizontal communication, the flow of suggestions is parallel between people at the same level. 
  
(D) All of these: Incorrect. Only parallel communication is correct in this context. 
Step 3: Conclusion. 
  
The correct answer is (C) At parallel as horizontal communication occurs between peers or colleagues.