From the graph we can conclude that highest temperatures are during July and August. These are months when the summer is on northern hemisphere. This excludes answer 2).
By observing temperature graph we can see that temperature varies greatly. Temperature in December, January and February is negative while in all other months it is positive. This excludes answer 1).
Answer 4) 90N is north pole. At north pole temperatures are much lower than those shown on the graph.
The correct answer is 3) 50N.