The answer is (C) Removal of non-functional Introns.
The process of protein synthesis begins with synthesis of RNA from gene. At this stage the RNA has both the introns and the exons. The removal of non-functional introns takes place in RNA to form mRNA that has all the coding exons. This makes it a pre-translational change. The protein is synthesised by the help of ribosomes that read the mRNA. This protein after translation can undergo various modifications like the addition of Acetate, removal of Methionine and the addition of a Disulfide Bridge.