Which of the following methods is used for extracting DNA from biological samples?

The method used for extracting DNA from biological samples is polymerase chain reaction (PCR). PCR is a technique that allows for the amplification of specific DNA sequences, enabling researchers and forensic scientists to obtain sufficient quantities of DNA for analysis from very small samples. This is especially important in forensic science, where biological evidence may be limited or degraded.

PCR works by mimicking the natural process of DNA replication but in a laboratory setting. It involves several cycles of heating and cooling that allow for the denaturation of the DNA, annealing of primers to the target sequences, and extension of the new DNA strands. This results in exponential amplification, making it possible to create millions of copies of a particular DNA segment.

While microscopy and X-ray crystallography are valuable techniques in their respective fields, they do not relate to the extraction or amplification of DNA. Microscopy is primarily used for visualizing small structures or organisms, and X-ray crystallography is used to determine the atomic structure of crystalline materials. Forensic analysis of written documents involves examining handwriting or ink, unrelated to DNA extraction and analysis. Thus, PCR is the correct choice as it directly pertains to the extraction and analysis of DNA in forensic applications.