To solve this problem, we need to determine which part of a diseased plant should be used in tissue culture to obtain virus-free healthy plants.$\newline$Let's analyze each option:$\newline \bullet$Option 1: Apical meristem only$\newline$ - The apical meristem is a region of actively dividing cells at the tips of roots and shoots.$\newline$ - It is often free from viruses because viruses typically do not reach these rapidly dividing cells.$\newline$ - However, using only the apical meristem may not be sufficient for all cases.$\newline \bullet$Option 2: Palisade parenchyma$\newline$ - Palisade parenchyma is a layer of cells found in leaves, primarily involved in photosynthesis.$\newline$ - This tissue is not typically used for obtaining virus-free plants as it can be infected by viruses.$\newline \bullet$Option 3: Both apical and axillary meristems$\newline$ - Axillary meristems are found in the axils of leaves and can also be free from viruses.$\newline$ - Using both apical and axillary meristems increases the chances of obtaining virus-free plants.$\newline$ - This is the most effective method for ensuring the plant is free from viruses.$\newline \bullet$Option 4: Epidermis only$\newline$ - The epidermis is the outermost layer of cells in plants and can be infected by viruses.$\newline$ - It is not suitable for obtaining virus-free plants.$\newline$Based on the analysis, the correct option is Option 3: Both apical and axillary meristems.