After transcription begins, several steps must be completed before the fully processed mRNA is ready to be used as a template for protein synthesis on the ribosomes. Which three statements correctly describe the processing that takes place before a mature mRNA exits the nucleus?
a. A translation stop codon is added at the 3' end of the pre-mRNA.
b. A cap consisting of a modified guanine nucleotide is added to the 5' end of the pre-mRNA.
c. A poly-A tail (50-250 adenine nucleotides) is added to the 3' end of the pre-mRNA.
d. Coding sequences called exons are spliced out by ribosomes.
e. Noncoding sequences called introns are spliced out by molecular complexes called spliceosomes.

a. A translation stop codon is added at the 3' end of the pre-mRNA.

c. A poly-A tail (50-250 adenine nucleotides) is added to the 3' end of the pre-mRNA.

d. Coding sequences called exons are spliced out by ribosomes.

Explanation:

In eukaryotes, during protein synthesis, the RNA must exit nucleus through the pores on the nuclear membrane and enter into the cytoplasm to undergo translation.  

The exit processes of mRNA are: the 5’ capping, 3’ Poly-A tail, and pre-mRNA splicing.

The 5’capping is the initial step with addition of 7-methylguanosine cap to the 5’ end of the mRNA by phosphate linkage. This process protects the mRNA from degrading and helps to initiate translation processes.

The 3’ Poly-A tail step is the cleaving of pre-mRNA and addition of addition of around 200 A nucleotides or the 3’poly (A) tail to the mRNA by the poly (A) polymerase enzyme protein complex. This prevents pre-mRNA degradation, facilitates movement of mRNA to the cytoplasm, and initiates translation.

Pre-mRNA splicing is done to remove the introns from the mRNA because they are not part of function protein coding. The exons which take part in protein coding sequence are spliced once the introns are excised.  

Finally, the resultant mRNA will exit the nucleus and enter the cytoplasm to undergo translation .