The correct option is A The boiling water was poured into the tea-pot.
Option A is the correct Passive voice. The rules for Passive form state that in the Simple Tense, the Past Participle (poured) is preceded by the verb was/ were in the simple past. Thus, "Now pour the boiling water into the tea-pot" will change to "The boiling water was poured into the tea-pot" in the passive voice.