What can be a correct way to set an observer to change UI in case if data was changed?

As an example. In an Activity we have our TimerViewModel object (extended ViewModel), named mTimerViewModel. mTimerViewModel.timer method returns a LiveData<Long> value.

What can be a correct way to set an observer to change UI in case if data was changed?
A . mTimerViewModel!!.timer.value.toString().observe (Observer { aLong -> callAnyChangeUIMethodHere(aLong!!) })
B . mTimerViewModel!!.timer.observe
(this, Observer { aLong -> callAnyChangeUIMethodHere(aLong!!) })
C . mTimerViewModel.observe
(Observer { aLong -> callAnyChangeUIMethodHere(aLong!!) })

View Answer

Answer: B