Answer:
Dad has a secret fishing spot
Step-by-step explanation:
A clause, a group of word that have at least a subject and a verb, can be either dependent (when it can not stand on its own because it does not express a complete thought), or independent (when the clause expresses a complete thought on its own, and does not need another clause in order to make sense).
In the sentence, "Dad has a secret fishing spot" is an independent clause because it has a full meaning on its own: If we see this clause alone, the clause expresses a complete thought. On the other hand, "where he catches the most fish." is a dependent clause because if it were alone, without another clause, it would not make sense.