A.
When I was answering the exam questions I was such nervous that I could feel butterflies in my stomach.
B.
When I was answering the exam questions I was so nervous that I could feel butters in my stomach.
C.
When I was answering the exam questions I was so nervous that I could feel butterflies in my stomach.
D.
I was answering the exam questions I was so nervous that I could feel butterflies in my stomach.