To solve this problem, we need to identify which tissue is responsible for producing secondary xylem and phloem in dicot stems.$\newline$Let's analyze each option:$\newline \bullet$Phellogen:$\newline$ - Also known as cork cambium, phellogen is responsible for producing the periderm, which includes cork cells.$\newline$ - It is not involved in the production of secondary xylem and phloem.$\newline \bullet$Vascular cambium:$\newline$ - This is a lateral meristem that produces secondary xylem (wood) and secondary phloem.$\newline$ - It is the main tissue responsible for the increase in thickness of stems and roots in dicots.$\newline$ - Therefore, vascular cambium is the correct answer.$\newline \bullet$Apical meristems:$\newline$ - These are located at the tips of roots and shoots and are responsible for primary growth, not secondary growth.$\newline$ - They do not produce secondary xylem and phloem.$\newline \bullet$Axillary meristems:$\newline$ - These are responsible for the growth of lateral branches and do not contribute to the production of secondary xylem and phloem.$\newline$Based on the analysis, the correct option is Option 2: Vascular cambium.