I'm thinking the answer is RNA. Is that right?
2 Answers
$\begingroup$
$\endgroup$
Almost always DNA. While RNA is possible, it’s much harder to synthesize (and keep stable) and DNA results in a purely DNA result that won’t need to be replaced with DNA later.
$\begingroup$
$\endgroup$
PCR primers are short pieces of single-stranded DNA, usually around 20 nucleotides in length.
You can read more about PCR on Khan Academy or Wikipedia.