Any tips on how to get high marks for this section? what do we need to be careful about? How do we write/arrange the notes according to the questions (sarabande,march...etc)

What is question 3? Is it the composition?

Yeah, It is a question where you will be given the opening of a melody and then asked to complete it. The question may indicate the style of the melody, for example by asking to continue a march, a gigue or a lullaby. Based on the style of melody given, how should I write a suitable melody for the question given?

I was told that you shouldn't be able to tell where the given opening finishes and your work starts.
As a starting point I'd think about rhythm. As in earlier grades where you are asked for an answering rhythm sketch in the rhythm and see that it fits with the given opening. If the opening has notes no shorter than crotchets following with bars full of semiquavers is unlikely to match the style.

For the more 'free' option you need to pick out the rhythmic or melodic feature and make use of it in your work.

Thanks for the information, Sbhoa :-D

There is another question I'm curious about.
When we are doing harmony. why consecutive 5th and 8ve are not allowed?

Easiest way to find out is to write them in and play it back if you are a pianist....
They tend to sound odd but I think it's a matter of learning certain conventions in 4 part harmony.