Final answer:
Introns are removed by RNA splicing in the nucleus through the action of spliceosomes, which are composed of proteins and snRNAs.
Step-by-step explanation:
Introns are removed from pre-mRNA through the process of RNA splicing that takes place in the nucleus. This critical step in mRNA processing is performed by protein and RNA complexes known as spliceosomes. Moreover, spliceosomes include small nuclear RNAs (snRNAs) which help recognize the sequences at the 5' and 3' end of the intron to ensure precise splicing. Therefore, the correct answer is B. RNA splicing in the nucleus.