Notice vs. pay attention

"Notice" suggests "becoming aware of". You can ignore something after you've noticed it. "Pay attention to" in this context instead means maintaining attention beyond first noticing the instructions — that is, they want you to keep the requirements in mind and follow the instructions.

At least in the US, it would be idiomatic to say "please note the following requirements", which implies reading them and also following them.

"Write" is what you do to the written product. You write a book, or a note, or a letter. "Write down" is what you do to material which exists in your mind but is not currently written. "Write down this phone number", "I have this great idea for a book": "Well, write it down!" And one more: "write down a recipe" means to record a recipe that you already know, but "write a recipe" implies creating the recipe as you write it.

The options are fine. The answer is (b) since it's a command, just like the other parts of the list: "write", "attach", "enclose", "send". But there's more to fix:

Before applying for a job, please note the following requirements: write your application letter neatly, attach your latest CV, [...] a copy of your ID card and a recent close-up photo, and send them to us in a well-sealed envelope.