Final answer:
C. Riboswitches are regulatory RNAs that alter translation responses to environmental conditions, by changing mRNA conformation in the presence of specific substances. Unlike microRNAs and siRNAs, riboswitches act directly without involving the RNA-induced silencing complex.
Step-by-step explanation:
The regulatory RNAs that alter translation in response to environmental conditions are riboswitches (option C). Riboswitches are segments of mRNA that can change their conformation in response to the presence of specific metabolites or ions, thereby influencing the expression of the genes they are associated with. Unlike miRNAs and siRNAs, which are involved in the RNA-induced silencing complex (RISC), riboswitches directly respond to changes in the cellular environment without the involvement of RISC.
Therefore, while microRNAs (option D) and small interfering RNAs (option B) participate in gene regulation and can lead to gene silencing by affecting mRNA stability through mechanisms involving RISC, they do not directly alter translation in response to environmental signals as riboswitches do. Transfer RNAs (option E) are involved in protein synthesis and are not regulatory RNAs. miRNA-induced silencing complexes (option A) involve miRNAs but are not known for environmental response modulation.