A unit rate is a rate given for one unit of an specific thing.
For example when we say that one ticket for the cinema costs $3.5 we are saying that that the unit rate is $3.5 per ticket.
So, to solve this problem, we should look for the one that explicitly relates something to only one unit.
Option A refers to 2 pens
Option B refers to 2 hours
Option C refers to 3 months
Optiond D refers to 1 week
So the unit rate is the option D.