Dropdown Restriction

How am I'm going to hide the "Discontinue" drop down if the user selected the process appendix? I already have variable for sub category with select box type

When "Process Changes Appendix" is selected, the following field is displayed:
* "Subcategory", dropdown: (a) Create New / (b) Discontinue / (c) Amend / (d) Download

When "Process Appendix" is selected, the following field is displayed:
* "Subcategory", dropdown: (a) Create new / (b) Amend / (c) Download