Final answer:
To change the sentence into passive voice: 'I shall think about the matter once I reach Pune', it can be transformed into 'The matter shall be thought about once Pune is reached'.
Step-by-step explanation:
The sentence 'I shall think about the matter once I reach Pune' can be changed into passive voice as 'The matter shall be thought about once Pune is reached'.
In the passive voice, the subject of the sentence receives the action instead of performing it. In this case, the subject 'I' becomes the object 'The matter', and the verb 'shall think' changes to 'shall be thought'. The phrase 'once I reach Pune' is transformed to 'once Pune is reached'.
Here, the primary topic is converting an active sentence into passive voice in English grammar.