No links: Which of the following statements best describes siRNA?

A portion of rRNA that allows it to bind to several ribosomal proteins in forming large or small subunits
A short double-stranded RNA, one of whose strands can block gene expression
A double-stranded RNA that is formed by cleavage of hairpin loops in a larger precursor
An RNA sequence that can block the expression of some transposons

A short double-stranded RNA, one of whose strands can block gene expression

Explanation:

SiRNA, also known as short interfering RNA, is are short double stranded DNA that binds to mRNA for the purpose of destroying the mRNA sequence, hence, preventing it from being translated into a protein.

According to this question, SiRNA can be described as a short double-stranded RNA, one of whose strands can block gene expression (translation of mRNA).

the answer is a or when it says crossing over