In vitro, free oligonucleotides, such as mrna transcripts, have a short half-life in the cytosol. what mechanism is used to stabilize these molecules?

POLYADENYLATION  mechanism.

Explanation:

This is the mechanism of adding a poly(A)tail to a mRNA to protect  the RNA.This tail is made up of adenosine monophosphate(AMP) compound, and therefore consists of only this purine base stretch.(adenine is purines).

Polyadenylation participate in production of mature mRNA for the process of translation in the cystosol, therefore ensuring stability.

However this tail shortened with time, and in extreme shortness the mRNA is degraded by enzymes in the cells.

Conclusively, the protection provided by the poly(A) tail

and production of mature mRNA  ensured the stability of these molecules