Which of the following is not a key consideration when developing a piece of writing?
a. the purpose of the message
b. the intended audience
c. the timeless of the message
d. the format of the message

I would say c. timelessness of the message.

Why? Because:

a) your piece of writing has to have a message - if there is no message, there is no point in writing it.
b) you have to have an intended audience you are writing for - it is not the same writing something for your friends, little brother/sister, professor or a potential employer.
c) the format of the message is important because of the previous factor and because the format usually affects the reception of the writing; nobody will be interested in your text if you write it without commas, fullstops, capitalized letters at the beginning of the sentence, or for example, without spaces between the words because it would be too confusing.

While, the timelessness is not as important. You are writing it for a special purpose now, and if it is good it might become timeless. But there are probably many great works (poems, scientific papers, novels, etc.) that were lost or forgotten, but that does not mean they were not great or significant.

Hope my answer is not too confusing :)