1. I like paris,.........
a.where my sister is living
b.in which my sister is living
c.which my sister is living in 
d.all are correct
2. I'd like to see the trees...you've picked these apples
a.from those trees
b.from which
c.which 
d.from that
3.My parents....send their best wishes
a.that you met last month
b.who you met last mont
c.whom you met last month
d.all are correct
4.Do you know the boy ....borrowed that book?
a.who
b.that
c.a và b
d.whom