"According to the forecast, it is very hot in Seoul."
Translation:예보에 의하면 서울에 아주 더워요.
The answer is not natural expression. 서울에 should be changed with 서울은. Or more natural expression is 예보에 의하면 서울은 아주 덥데요.
Maybe, but notice they're not saying that Seoul is hot in English, they're saying it is hot IN Seoul, thus the -에 as opposed to -은
For all wondering what this -데요 grammar BUis2 used, here's a link: