Scientists use polymerase chain reaction (PCR)

Question

to amplify a segment of DNA into many copies.
to insert new nucleotide segments into a plasmid.
to make a complementary strand of DNA.
to link together multiple fragments of DNA

WHICH ONE WOULD YOU PICK AND MOST IMPORTANTLY, WHY?

The first answer is the correct one. PCR is done to provide copies of DNA segments (genes) from small collected samples. Larger amounts are generally needed for DNA testing.
See http://users.ugent.be/~avierstr/principles/pcr.html