it's in the present tense, but the present perfect is more fitting translation in English. That's because the literal translation is "The clock stands" which means that it has stopped in the past, not at the present moment. If you want to say that the clock stops I would say "Kello pysähtyy".
While seisoi is the active simple past sg. 3rd yes-form of the verb seisoa, the exercise has Kello seisoo, i.e. the active present sg. 3rd yes-form. I trust Anna839191 here, the perfect is more fitting in English.
Note, Duolingo does not take up past tenses (and good so).
Advanced content warning:
First the verb seisoa shows a state, not a change to that state. As Anna839191 said, if you want to say that the clock stops, i.e. enters the stopped state, use pysähtyä.
The tenses in Finnish often have two nuances that may not be obvious to all:
- has the action ended?
- is this new information?
Let us see how they work in the four tenses of the indicative mood:
- Kello seisoo [preesens] : new info – the clock stopped in the past and still is in that state
- Kello seisoi [imperfekti] : new info – the clock stopped in the past but is no longer in that state
- Kello on seissyt [perfekti] : old info – the clock stopped in the past and still is in that state
- Kello oli seissyt [pluskvamperfekti] : old info – the clock stopped in the past but is no longer in that state
Note, in case of the two last sentences, those with the old info, the listener usually expects to get some additional, new info. For instance:
- Kello on seissyt viime viikosta lähtien : …since last week.
- Kello oli seissyt viikon, ennen kuin vaihdoimme patterit : …a week before we changed the batteries.
How to map these into English is the tricky part. To me the selected "The clock has stopped" sounds ok for Kello seisoo.