The RNA which does not exhibit intramolecular  (intrachain) base pairing isGroup of answer choicesrRNAtRNAmRNAboth mRNA and 18s RNAnone of these

To answer the question regarding which type of RNA does not exhibit intramolecular (intrachain) base pairing, we should consider the characteristics and functions of the different types of RNA.

1. rRNA (ribosomal RNA): rRNA is a fundamental component of ribosomes, which serve as the site of protein synthesis. While rRNA can have complex secondary structures, it typically exhibits intrachain base pairing that helps stabilize its structure.

2. tRNA (transfer RNA): tRNA has a well-defined structure with anticodons that base-pair with mRNA during translation. The cloverleaf shape of tRNA is due to intrachain base pairing, therefore, it does exhibit intramolecular base pairing.

3. mRNA (messenger RNA): mRNA is responsible for conveying genetic information from DNA to the ribosome, where it serves as a template for protein synthesis. Unlike rRNA and tRNA, mRNA generally does not fold into a structured form involving extensive intrachain base pairing; it tends to remain linear.

4. 18S rRNA: This is a component of the small subunit of the eukaryotic ribosome and, similar to other rRNAs, exhibits intrachain base pairing to maintain its secondary and tertiary structures.

5. Conclusion: Among the listed options, mRNA is notable for the lack of significant intramolecular base pairing, as it generally exists in a linear form during translation.

Final Answer

The RNA which does not exhibit intramolecular (intrachain) base pairing is mRNA.