To solve this problem, we need to identify which cell organelle is enclosed by a single membrane. Let's analyze each option:$\newline \bullet$Mitochondria:$\newline$ - Mitochondria are double-membraned organelles. They have an outer membrane and an inner membrane, which is folded into cristae.$\newline$ - Therefore, mitochondria are not enclosed by a single membrane.$\newline \bullet$Chloroplasts:$\newline$ - Chloroplasts are also double-membraned organelles. They have an outer membrane and an inner membrane, with thylakoid membranes inside.$\newline$ - Therefore, chloroplasts are not enclosed by a single membrane.$\newline \bullet$Lysosomes:$\newline$ - Lysosomes are single-membraned organelles. They contain digestive enzymes and are involved in breaking down waste materials and cellular debris.$\newline$ - Therefore, lysosomes are enclosed by a single membrane.$\newline \bullet$Nuclei:$\newline$ - The nucleus is enclosed by a double membrane known as the nuclear envelope, which has an inner and an outer membrane.$\newline$ - Therefore, nuclei are not enclosed by a single membrane.$\newline$Based on the analysis, the correct answer is Option 3: Lysosomes, as they are the only organelles listed that are enclosed by a single membrane.