How can I change this to passive voice?
Honey is sweet
How can I change this to passive voice?
Honey is sweet
It's not possible to directly change the example sentence into the passive voice, because there is no transitive verb that's being used in the first place.
However, the most expedient way of making it passive is to simply insert a transitive verb:
Honey is made sweet.
If the agent of the production of honey (and its sweetness) is bees, then you'd have the following.
Tell your test giver that they've given you an impossible question.
Let's look at a sentence that can be turned into passive voice.
Mark (noun) drove the car (noun).
The car (noun) was driven by Mark (noun).
Let's apply this to your example sentence.
Honey (noun) is sweet (adjective).
Sweet (adjective--problem here) was been honey [by ????].
Adjectives can't be the subject of sentences. So turning it into passive voice without modification is impossible.
Copular verbs can't be passive, because they are basically being used as an "equal sign" to say X is the same as Y, or say what an attribute of Y is, and therefore there's no agent and no possibility that there can be an agent.