Introns are DNA sequences that:
A. Code for functional domains in proteins.
B. Are removed from pre-mRNA by spliceosomes.
C. Allow one gene to make different gene products depending on which introns are removed during splicing.
D. Both (B) and (C).

Hint: Introns are removed by the process of splicing from hn-RNA and then it converts into mature RNA. Prior to the RNA molecule being translated into a protein, introns are non-coding regions of an RNA transcript or the DNA encoding it. Exons are the regions of DNA (or RNA) that contain the protein-coding information.

Complete step by step solution:
Presence of introns represents reminiscence of antiquity which means it shows resemblance with old creatures. They were present in archaebacteria and were absent in prokaryotes but showed up again in eukaryotes. The ability to produce numerous proteins from a single gene thanks to alternative splicing is the most crucial function of introns. After being spliced, some introns undergo further processing that allows them to encode functional RNA molecules.
From this angle, introns serve a significant function. In the creation of novel exon combinations, they act as recombination hotspots. In other words, they are present in our genes because they served as a quicker method of gene synthesis during evolution. Spliceosomes cut off introns, and as the tropomyosin example shows, differential splicing can result in various gene products. Intron-containing DNA loops were observed in hybridization tests using mature messenger RNA rather than pre-mRNA.
So, option (D) is correct.
Note:
Hn-RNA stands for heterogeneous nuclear RNA. Just after the process of transcription hn-RNA is formed which undergoes post transcriptional processes in order to form mature RNA. Mature RNA differs from hn-RNA in having a cap and a tail, also the process of splicing occurs in which introns are removed by spliceosomes and exons exist in the mature RNA.