Which technique is most useful if the DNA sample is very small?

Correct answer: PCR

The direct answer is A: PCR is most useful when the DNA sample is very small. PCR, or polymerase chain reaction, is an in-vitro method that amplifies a selected DNA region. Its basic cycle is: denaturation separates the two DNA strands; primers attach to the target boundaries during annealing; a thermostable DNA polymerase extends the primers during extension. Repeating the cycle produces approximately exponential amplification of the selected region, provided suitable primers and a usable template are present. Option A is correct because even a small starting amount can yield many copies for testing. Option B, simple filtration, separates particles but does not copy DNA. Option C, boiling, may separate strands or damage biological material but does not specifically amplify DNA. Option D, drying, removes water and does not create DNA copies. Memory cue: PCR is a DNA photocopier, not a filter or heater.