On one occasion he persuaded me to accompany him on a shooting expedition he ( was planing).

On one occasion he persuaded me to accompany him on a shooting expedition he was planning

1. has planned
2. had planned
3. planned
4. No improvement

Explanation
. (b) Past Perfect Tense (had planned) will be used.
Sentence structure is as follows :
Subject + Aux.V. + Main V. + Object + he had planned _____
Before we arrived at school the class had started

Second action First action
Hence, had planned is the right usage.