(i) The gobar gas obtained by processing dung is used as fuel.
The sentence is in passive voice(as can be seen with the help of the word ‘by’)
If we write the sentence beginning with people, it would transform into an active voice.
The gobar gas i.e. the subject will become the object.
New Sentence formed: People process dung to obtain gobar gas and use it as fuel
(ii) The consumption of milk in both urban and rural areas has risen sharply.
Past perfect tense is used to indicate the actions that took place before some point of time in the past. It uses the verb i.e. had + past participle(V3).
Example: He had visited London during his childhood.
New Sentence formed: The consumption of milk in both urban and rural areas had risen sharply.
(iii) The milk processing industry is a small one.
A complex sentence is a sentence with one independent clause and at least one dependent clause. In complex sentences, dependent clauses are linked to independent clauses with subordinating conjunctions.
Example: Because my coffee was too cold, I heated it in the microwave.
New Sentence formed: The industry that processes milk is a small one.
Final Answer:
Upon rewriting the (i) beginning with people, it becomes People process dung to obtain gobar gas and use it as fuel, (ii) using the past perfect tense becomes The consumption of milk in both urban and rural areas had risen sharply. and (iii) upon changing into complex becomes The industry that processes milk is a small one.