What is thought to bring different organisms such as fish and dolphins to have evolved a similar body plan?

their very similar environments.

Explanation:

The type of evolution that is referenced here actually has a name -- convergent evolution. It's when two types of organisms/species have nothing in common ancestor-wise but still evolve similarly due to their similar environments. The similar body plan the question is referencing could include fins and other "fish features" as well. :-)

c.) polymerase chain reaction

- "unlike the original dna fingerprinting method, dna profiling does not use restriction enzymes to cut the dna. instead it uses the polymerase chain reaction (pcr)? to produce many copies of specific str sequences. pcr is an automated procedure that generates lots of copies of a specific sequence of dna."

as many times as you want