Concept:
A memorandum (memo) is used for communication within an organization.
Step 1: Understand usage.
\[
{Memo} \Rightarrow {internal communication within company}
\]
Step 2: Evaluate options.
(A) $\times$ external communication
(B) $\times$ legal documents
(C) \checkmark internal communication
(D) $\times$ advertising purpose
Hence, the correct answer is Internal company communication.