If you want to reach certain categories of apps with in-app display ads, or if you know the apps you want to target, you should:

Q- If you want to reach certain categories of apps with in-app display ads, or if you know the apps you want to target, you should:
Ans:

Enable Active View reporting
Create a Display Network campaign targeted to mobile apps
Go to “Campaign Exclusions” from the Display Network tab
Create a placement inclusion for mobile apps