Final answer:
The correction that should be made is to insert a comma after tour.
Step-by-step explanation:
The correction that should be made to the sentence is option c) Insert a comma after tour. The sentence is a compound sentence, and a comma is needed to separate the two independent clauses. The two independent clauses are: 'At first I was skeptical about the visit because I had never been interested in horticulture before' and 'midway through the tour I discovered something new that really sparked my interest: Japanese gardening and the ancient art of growing bonsai trees.'