Translation:It is hot in the summer.
When given a "choose one, to fill in the blank", both さむい And あつい make gramatical sense, (and is that not what this type of question is testing?), yet my curious mind gets a sour "wrong!" for expressing the grammatically acceptable なつはさむいです. There are places in the world where this is perfectly true.
To be fair, "cold" isn't exactly an objective unit of measurement. It's based in perception. If you're acclimated to a hotter climate, summer can be cold to you elsewhere. When someone I know moved to America's northwest from Tennessee, she was shivering in a sweater in the middle of summer.
So, I agree: "Summer is cold" is still grammatically correct, and even can have practical application, as it is expressing a perception rather than a precise measurement of temperature (i.e., 47 degrees C or what have you.)
Person1, who is used to the local temperatures: Summer is hot!
Person2, who came from somewhere sweltering: No, it isn't! Summer is cold!
Duo: I take my clothes off in winter.