"Yes, it is necessary."
Translation:Ja, det är nödvändigt.
My understanding of the difference between jo and ja has to do with whether or not the question is asked as a negative. If the question is in the negative then you answer jo. If the question is in the positive you answer ja. Är det din bil? Ja. Är det inte din bil? Jo. There's also the difference if you're uncertain about your answer. If you're certain about the answer in the affirmative you say ja, but if you are uncertain you can answer jo. Vet du honom? Ja. (meaning yes I know him) or Jo (meaning maybe I do know him, but I'm unsure).