A: Those knits are amazing! You’re so talented. You should sell them!
B: Thanks! But I really enjoy doing it, so if I turned it into a job, I’m afraid I wouldn’t find it fun anymore.
A: That’s a valid concern.
In this situation, what should you say ‘that’s a valid concern’ in Japanese?
Casual
A：確かに。
Note: This is not a word-for-word translation.
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A word-for-word translation
A：それは正当な懸念です。
According to the Google Translation
A：それはごもっともですね
Although それは正当な懸念です is grammatically correct and sounds natural as a sentence, it is overly formal in this situation. The same can be said of それはごもっともですね.
それは正当な懸念です is appropriate when speaking on the news, for example.
それはごもっともですね is often used when apologising to a business partner, for example, to express agreement with what they have said.
In both cases, それは is often omitted.
In this context, I think 確かに is the best option.
In situation between close relationships, you can use ごもっとも in a humorous way.
This indicates more agreement than 確かに.
Kotoba no Hibiki 