Final answer:
In RNA, thymine is the base not linked to ribose sugar; it is replaced by uracil. RNA is primarily single-stranded and has a backbone constructed of ribose units connected by phosphodiester linkages.
Step-by-step explanation:
In a strand of RNA, the base that is not found covalently linked to a ribose sugar is thymine (4). RNA is primarily single-stranded and has a backbone constructed of ribose units connected by phosphodiester linkages. While RNA contains adenine, cytosine, and guanine, similar to DNA, thymine is replaced by uracil (U) in RNA. Therefore, in RNA, adenine pairs with uracil, which possesses a structure similar to thymine but lacks a methyl group.