I like these exercises where you choose between the different forms of a word, because that helps me to associate them and remember both conjugations (or singular and plural forms, or etc.).
The reason would be because 'er is zingen' is not a passive voice sentence. (Btw, as I'm a learner I cannot be 100% sure about it, but I believe that your sentence is not a correct sentence to begin with, sorry to say).
In order to make a passive voice sentence, you need to use:
• (A form of worden)+(past participle)
Er wordt gezongen= There is singing
Er werd gezongen= There was singing
• (A form of zijn)+(past participle)
Er is gezongen= There has been singing
Er was gezongen= There had been singing
Of course, the English translations are not 'real' passive voice sentences, but the Dutch impersonal passive voice seems to be quite tricky to translate into English. (By impersonal passive voice here I mean the passive voice that takes er as a grammatical subject).
For further information, please check:
Hope this helps!