Even though most of the plants and animals have a large number of transposable elements (TE) in their genomes, they are able to survive due to the following reasons:
• Most of the TEs are inactive due to mutations. A mutation in a TE does not affect the phonotype of the organisms and thus, the TEs accumulated mutations over time. Thus, they are inactivated.
• Host cells employ several mechanisms to inactivate the TEs. RNAi (Ribo nucleic acid silencing), PiRNA (piwi-Argonuate RNAi), CrRNA (clustered regularly interspaced short palindromic repeats RNA) of bacteria, are all examples of mechanisms for silencing TEs.
• Safe heavens are regions in the introns which are preferred sites for the TEs for insertion. Introns are non-coding regions of genes. Insertion of TEs into the introns does alter the gene expression. Thus, the TEs survive in the cell without being getting eliminated.
Hence, the correct answer is option (d).
The options (a), (b), and (c) are all true in case of survival of plants and animals through so many TEs are present. Each of these options alone is not responsible for survival of the organisms.