Which modification is crucial in preparing an mRNA strand for proper translation? removal of introns and the splicing together of exons joining of ribosomal subunits to form a ribosomal complex removal of exons and the splicing together of introns addition of a poly-A tail addition of a poly-U tail

Removal of introns and the splicing together of exons.

Introns are regions of the DNA code that do not code for amino acids; therefore, they need to be removed before the mRNA strand is translated. Exons are the parts of the strand that do code for proteins, so they need to be pieced together once the introns are removed.