Answer:
The correct answer will be- DNA primers
Explanation:
The polymerase chain reaction is a widely used technique which replicates the DNA from a small sample of DNA in in-vitro conditions.
The mechanism of the Polymerase chain reaction is similar to the DNA replication mechanism which requires a DNA polymerase enzyme which in PCR is Taq polymerase and Primers which assists the DNA polymerase to begin the replication.
Since in the given question, the scientist added the DNA polymerase enzyme but forgot to add the single-stranded DNA called primers.
Therefore, primers are the correct answer.