Just a note to potential downvoters and close voters, even though this question by itself is on topic, the reason that this being a core theme, make this even more on topic
EDIT
As you are making use of the twentyfourteen theme, you have to remember this one golden rule ( and this applies to all themes and plugins that you are not the author of ), you should not make changes to the theme ( or plugin or core files ) directly. You should create a child theme. The page in the link actually handles this part perfectly as it specifically targets the twentyfourteen theme
ORIGINAL ANSWER
You can solve this by creating a category template specific to the category you want to target
Have a look at the Template Hierarchy. Let’s imagine you have a category with a slug one and you need to only add your box to this category, you can create a template called category-one.php
Now, whenever the one category is selected, category-one.php will be used. Any other category will use the normal category.php template
Alternatively, you can wrap your function with a is_category( 'one' ) conditional statement when adding your box. This will make sure that only category one is targeted