To solve this problem, we need to identify the organism in which the experimental proof for semiconservative replication of DNA was first demonstrated.$\newline$Let's analyze the historical context:$\newline \bullet$The concept of semiconservative replication of DNA was first demonstrated by Matthew Meselson and Franklin Stahl in 1958.$\newline \bullet$They conducted their experiment using the bacterium $\textit{Escherichia coli}. \newline \bullet$In their experiment, they used isotopes of nitrogen $(^{15}N$ and $^{14}N)$ to label the DNA and demonstrated that each daughter DNA molecule consists of one old and one new strand.$\newline$Therefore, the experimental proof for semiconservative replication of DNA was first shown in a bacterium.$\newline$This corresponds to Option 2: Bacterium.