Passive form of the sentence

Sour is the taste of the orange.

Here's how passive voice works.

1) Move the subject (in this case, orange) to the end or near the end.
2) Move the object or adjective (in this case it's an adjective, sour) to the start or near the start. In this case, you don't need an article before the word, probably because it's an adjective.
3) Add a form of the verb to be, between the object and the subject. In this case, since it's third person singular, you use the form is.
4) Add articles as necessary. Here, you need a definite article in front of the word taste.
5) Add a period at the end of the sentence. Now, you could technically write and say a passive voice question, but those are rare although technically correct. Most native speakers would find it odd and nonidiomatic to say "The taste of the orange is sour?" But you technically could.

In your sentence, “Tastes” is a verb already in adjective passive.