At first I was going to say my favorite tropical rainforest animal is the toucan, but second thought, my favorite is the tapir.

At first I was going to say my favorite tropical rainforest animal is the toucan, but on second thought, my favorite is the tapir.

Explanation:

We must fill in the blank with a preposition that completes the phrase used. In this case, we are using a fixed expression: "on second thought." Thus, no other preposition would be possible here, since the phase is always "on second thought."

"On second thought" is used to indicate that we have changed our opinion or decision about something after thinking about it again. For example: "She was going to order some chocolate ice cream. But, on second thought, she went for the mint one."

you have to tell us what you're talking about or it's impossible to answer.

im not sure about this one