Step 1: Understand fin efficiency.
For an insulated tip, the fin efficiency is a function of the thermal conductivity, length of the fin, and heat transfer. The formula for fin efficiency with an insulated tip is:
\[
\eta = \frac{\tanh(ml)}{ml}
\]
Where \( m \) is a function of the thermal conductivity and cross-sectional area, and \( l \) is the length of the fin.
Step 2: Conclusion.
The correct formula for fin efficiency for an insulated tip is \( \frac{\tanh(ml)}{ml} \).
Final Answer:
\[
\boxed{\frac{\tanh(ml)}{ml}}
\]