need guideline for the essay
 

can any one of you guide me how to write essay on the topic " if gold rusts what shall iron do"

Why would you choose a topic which is totally different to your 'Optionals'? I'd suggest to pick the one you know most about. Rest is only the art of putting information, analysing the contents and concluding all of it in a short paragraph.

Essay isn't difficult, rather technical.

Fpsc has changed the rules. They need future policy makers to bring Pakistan out of troubles. The requirements for essay need research based work. So they are preferring issues rather than a creative work to caliber the talent of an aspirant. You can check 2016 paper too. Such topic " if gold rusts what shall iron do" can increase your creativity but holds no criteria to pass the exam and it is difficult to cover all aspects of the topic without a background knowledge in literature. By putting your efforts on such topics, you may placing yourself in a danger zone. Their numbers in an essay is restricted to 1/2 or incase of abnormality it may be 3.