The correct answer is long non-coding (lnc) RNA. Xist lncRNA coats the X-chromosome from which it is transcribed, effectively silencing that X-chromosome. MicroRNAs are small RNAs (~20 base pairs (bp)) and play a role in RNA silencing and post-transcriptional regulation of gene expression. Short interfering RNAs are double-stranded (20-25 bp) and play a role in post-transcriptional gene silencing. Piwi-interacting RNAs are small non-coding RNAs that interact with piwi proteins in epigenetic and post-transcriptional silencing of genetic elements such as retroposons. While MicroRNAs, siRNAs and Piwi-interacting RNAs all silence genes, the mechanism of X-chromosome inactivation requires Xist lncRNA.

Protein synthesis can be directly affected by molecules involved in translation, or indirectly by molecules involved in the transcription of mRNA templates.

Transfer RNA (tRNA) is involved in translation and serves the function of bringing amino acids to ribosomes. Due to its important function in translation tRNA, is capable of globally controlling translation and, therefore, is involved in protein regulation.

Long non-coding RNA (lncRNA) has been shown to regulate transcription in a number of ways. One of the most prominent is the existence of a lncRNA (Xist) that inactivates the majority of the extra X-chromosome in human females.

Micro RNA (miRNA) is involved in a process known as RNAi and is capable of controlling the translation of targeted molecules of mRNA.