How To Display Only 1 Category

I can’t seem to figure out how to make a portfolio page display only one category/section id/custom post type/whatever. I’ve tried assigning “Section IDs” and “Section Numbers” to specific portfolios, corresponding with names of portfolio categories and yet it displays all portfolio items, regardless of the category assigned to the portfolio item and the section id assigned to the portfolio group. How do I make my Graphic Design page only display items that have the “graphic design” category assigned to them?

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