Step 1: Understand the concept of grapevine communication.
Grapevine communication refers to the informal, often unofficial communication that occurs among employees. It can be based on rumors, personal exchanges, and social interactions.
Step 2: Evaluate the options.
- Grapevine communication is informal, not part of the formal communication channels.
- It is not written and not official.
Step 3: Conclude.
Grapevine communication is inherently informal.
Final Answer:
\[
\boxed{\text{Informal}}
\]