Es verano is completely correct. When you guys have ambigious test questions like this -- and there are MANY on this web site -- it gets frustrating.
"Es verano" is correct, but this particular lesson is about the colloquial use of the verb ESTAR rather than SER for days of the week, dates and seasons. This is emphasised in the lesson, hence the answers will have to reflect this colloquial usage.
Yes, as John said, this is a specific use of estar when talking colloquially about days of the week, seasons and dates. That doesn't mean that Es verano is wrong, it's simply not tested in this topic.
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Test your Spanish to the CEFR standard