To solve this problem, we need to identify which stain is used to visualize DNA fragments separated on an agarose gel. Let's analyze each option:$\newline \bullet$Option 1: Acetocarmine.$\newline$ - Acetocarmine is a stain used primarily for staining chromosomes in plant cells. It is not used for visualizing DNA on agarose gels.$\newline \bullet$Option 2: Aniline blue.$\newline$ - Aniline blue is a stain used for staining cell walls and other structures in plant tissues. It is not suitable for visualizing DNA on agarose gels.$\newline \bullet$Option 3: Ethidium bromide.$\newline$ - Ethidium bromide is a fluorescent dye that intercalates between DNA bases. It is commonly used to stain DNA in agarose gels, allowing visualization under UV light.$\newline \bullet$Option 4: Bromophenol blue.$\newline$ - Bromophenol blue is a tracking dye used in gel electrophoresis to monitor the progress of the run. It does not stain DNA.$\newline$Based on the analysis, the correct option is Option 3: Ethidium bromide, as it is specifically used for visualizing DNA fragments on agarose gels.