To solve this problem, we need to understand the role of vascular cambium in plant growth.$\newline$The vascular cambium is a type of lateral meristem responsible for secondary growth in plants. It produces secondary vascular tissues.$\newline$Let's analyze the options:$\newline \bullet$Option 1: Primary phloem.$\newline$ - Primary phloem is part of the primary growth and is produced by the apical meristem, not the vascular cambium.$\newline \bullet$Option 2: Secondary xylem.$\newline$ - The vascular cambium produces secondary xylem towards the inside of the stem or root.$\newline$ - This is part of the secondary growth and contributes to the thickening of the plant.$\newline \bullet$Option 3: Periderm.$\newline$ - Periderm is part of the protective tissue that replaces the epidermis during secondary growth, but it is produced by the cork cambium, not the vascular cambium.$\newline \bullet$Option 4: Phelloderm.$\newline$ - Phelloderm is also part of the periderm and is produced by the cork cambium, not the vascular cambium.$\newline$Therefore, the correct answer is Option 2: Secondary xylem.