Step 1: Understand the context. Siti hears thunder and makes a decision about the picnic. The action that follows is stopping or cancelling a planned event. Step 2: Match the context to the correct phrasal verb. 'Call off' = to cancel or abandon a planned event. Step 3: Eliminate the other options. 'Call up' = to phone or summon. 'Call on' = to visit or to ask someone to do something. 'Call out' = to shout or to publicly challenge. Step 4: Confirm the answer. Only 'call off' fits the meaning of cancelling the picnic. Answer: off