Which does not apply to the technique known as gel electrophoresis?
(a) The DNA is pulled to the positive end of the gel box.
(b) The agarose gel provides for a porous matrix.
(c) The buffer solution helps supply ions needed to conduct an electric current.
(d) The larger DNA fragments move further down the gel than the smaller ones due to their heavier weight.

Hint: Gel electrophoresis is a technique for separating DNA fragments based on their size. DNA samples are loaded into wells (indentations) at one end of a gel and pulled through the gel by an electric current. Because DNA fragments are negatively charged, they gravitate toward the positive electrode.

Complete answer:
Gel electrophoresis is a technique for separating DNA fragments (or other macromolecules like RNA and proteins) based on size and charge. Electrophoresis is the process of passing a current through a gel that contains the molecules of interest. The molecules will travel through the gel in different directions or at different speeds depending on their size and charge, allowing them to be separated from one another.
Agarose gel is used in this method to create a porous matrix through which macromolecules can pass. A buffer solution is used to provide the ions required to conduct an electric current. The DNA is kept near the gel box's negative end. Depending on its size, the electric current is drawn to the positive end after being applied. Smaller DNA fragments travel further down the gel than larger ones.
Each DNA molecule has the same charge per mass. As a result, gel electrophoresis of DNA fragments only separates them based on size. We can use electrophoresis to determine how many different DNA fragments are present in a sample and how large they are in comparison to one another. We can also measure the absolute size of a piece of DNA by comparing it to a standard "yardstick" made of DNA fragments of known sizes.
During electrophoresis, the movement of a charged particle in an electrical current, a gel is used as an anticonvective medium or sieving medium. Gels suppress the thermal convection caused by the electric field, and can also act as a sieving medium, slowing the passage of molecules; gels can also simply serve to maintain the finished separation so that a post electrophoresis stain can be applied.

Thus, the answer is option D: The larger DNA fragments move further down the gel than the smaller one due to their heavier weight.

Note: DNA Gel electrophoresis is typically used for analytical purposes, often following polymerase chain reaction (PCR) amplification of DNA, but it can also be used as a preparative technique before the use of other methods for further characterization such as mass spectrometry, RFLP, PCR, cloning, DNA sequencing, or Southern blotting.