Neither of them : 'tacky' means unattractive, unappealing ; the writer is not impressed by orchids, (s)he doesn’t like them. The word is usually used to describe something which could have been done differently, and is not applied to something like a flower. ’The restaurant was pretty tacky - it was a bit *grubby, service was poor, and the food was disappointing.’ [*dirty] ’We wanted to buy a souvenir of our visit but the stuff on sale in the gift shop was tacky.’

As Adrian says, absolutely none of these definitions is correct. However, if you are in a difficult position where you need to complete this exercise for a teacher, and you can't tell the teacher it's not correct, I'd choose 'conventionally beautiful'. This is only my guess because often things can be seen as 'tacky' when they are used too often. For example, someone might describe red roses for Valentine's Day as 'tacky'. So put that if, for some reason, you must complete this exercise. However, it is not really correct so, if you have the option, stop using this source for your language practice.