Concept: Sections 15 to 20 of CPC deal with place of suing. Section 18 specifically deals with cases where local limits of jurisdiction are uncertain.
Step 1: Understand local jurisdiction.
Local jurisdiction means the territorial area within which a court can entertain a suit.
\[
\text{Local jurisdiction} = \text{Territorial power of court}
\]
Step 2: Identify the relevant provision.
When there is uncertainty about the local limits of jurisdiction of courts, Section 18 CPC applies.
\[
\text{Uncertain local limits} \Rightarrow \text{Section 18 CPC}
\]
Step 3: Select the correct option.
Therefore, the correct answer is Section 18 CPC.