A mixture containing DNA fragments of sizes 150 bp , 450 bp , 900...

In agarose gel electrophoresis, DNA fragments are separated based on their size. Since DNA molecules are negatively charged, they migrate towards the positive electrode (anode) under an electric field.

The agarose gel acts as a sieve, allowing smaller DNA fragments to move faster and further through the pores of the gel matrix compared to larger fragments.

Among the given fragments of sizes $150\text{ bp}$, $450\text{ bp}$, $900\text{ bp}$, and $2500\text{ bp}$, the $150\text{ bp}$ fragment is the smallest. Therefore, it will travel the maximum distance and will be found closest to the anode after the run is completed.

Answer: The $150\text{ bp}$ fragment