the main subject is "Les émissions -de sport- "
les émissions is the plural of l'émission or une émission = feminine noun
so the adjective "affreux" has to be feminine/plural as well
Les émissions de sport sont affreuses is the correct sentence
if it had been "les reportages de sports sont affreux" it would have been correct because "le/un reportage" is masculine and affreux is ok for masculine singular and plural
affreux (masculine singular and plural)
affreuse (feminine singular) //affreuses (feminine plural)
hope it was clear enough to help you !!