The answer is a, but why can c be an answer as well? Is it because of "in that case"? c seems to have full qualification, so I can't understand.
W: Let's go on a hiking excursion tomorrow.
M: Again? We did that a couple of weeks ago
W:Yeah, but work has been stressful and fresh air would be invigorating.
M:________a__________________
a)OK, I'll research a good trail we can use
b)You're welcome to be my travel companion
c)In that case, we should get going.
d)I'm sure you'll manage the climb.
'We should get going' suggests now, but the suggested hike is tomorrow.
Context is always important; labelling is rarely important.