What is the right answer ?
Karl and Susan have agreed to come to our party, _______ has made Maria very happy.

a) that
b) which
c) who
d) what

I chose a). Could it be "which" ?

Question

What is the right answer ?
Karl and Susan have agreed to come to our party, _______ has made Maria very happy.

a) that 
b) which
c) who
d) what

I chose a). Could it be "which" ?

Answer

It should be "which". "That" requires either a period and a new sentence, or a semi-colon.

Answer

While Susan612 is right for all technical grammatical purposes, many people would say "that" while speaking, so if you're talking to someone and say "that" instead, it's perfectly okay. However if this is for a grammar class, then stick with "which".