The medicine <em><u>is</u></em> from a tropical plant.
I'm busy right now, but I'll be with you <em><u>in</u></em> a moment.
I am not sure about the last one but I think it is...
We <em><u>arrive</u></em> while I <em><u>get</u></em> in a music shop.
Honestly none of the options are past tense. I am going to correct it for you. It should be like this...
We <em><u>met</u></em> while I <em><u>worked</u></em><em> </em>in a music shop.