Answer:
C. blind to the truth
Step-by-step explanation:
The Duke is in love with Olivia, at first, who is in love with Cesario. The problem is that Cesario is actually Viola, disguised as a man. Viola is in love with the Duke and working for him as Cesario while being the messenger for the Duke when he wants to send messages to Olivia. The Duke is completly oblivious of the true nature of the whole situation. He believes that Olivia is mourning when she in fact loves Cesario.