Since it asked for Prime Minister, I assumed it was British, so I gave 1st & last name for UK's current one. Didn't think about Canada -- and I'm ashamed to say I don't recall (his?) name.
This Alzheimer's screening takes some items from typical mental status evaluations done in interviews by psychiatrists (& some psychologists) -- but it sets the bar at a pretty low level. It's commonly used to screen for the likelihood of that condition and to help decide whether to refer the patient for more extensive and precise tests by psychologists and neurologists.
If you don't know your date of birth or the name of the president of the US (a commonly used question in the US, equivalent to asking for the PM in a UK country) then you obviously are having some problems with reality contact. Since the 1940's, maybe earlier, common questions on US mental status exams have been
"Who is the president of the United States?" followed by
"Who was the president before him?"