If the word verdadeiro translates as "true" or "truthful", it seems the meaning could be that the clock is actually "running true", i.e., it tells the true time and is accurate. There is really no indication that we are talking about evaluating an antique or a set piece rather than discussing the functioning of the clock. Does the verb have any impact on that assumption?
You are right about "kosher" being interpreted as acceptable/alright. However, I'd like to suggest a better example than asking how someone is. Usually, "kosher" (originally a term referring to food prepared according to Jewish laws) means legitimate, something that is done according to specific rules. An example of that would be:
Friend A: "Hey, I just got an email from one of my contacts claiming that she's stuck in Bermuda with no money! She asked me to send her $1,500 asap. What should I do???" Friend B: "Hmmm... something about it does not look kosher to me!"
We use 'real' as to describe something as authentic in English and would be most likely to say "the watch is real" in real life. However, I think they want to us to understand other words and what is actually being said here. Like... I'm sure they would accept this answer in any other situation but just not for this lesson. =D
This is the second time I've gotten this one incorrect. the first time I put "the clock is real", which was counted incorrect, and I was given "the clock is legit" as the right answer. The second time I put "the clock is correct", and I was given "the clock is kosher" as the right answer. Wth, Duolingo, that makes no sense. I flagged it, and I'm leaving this comment here.