Answer: C) Negative PNA
Step-by-step explanation: PNA refers to the Pacific-North American tele-connection pattern which is responsible for the variations occurring in the atmospheric pressure and thus changing the atmospheric circulation. The natural variation in the climate can be in positive or negative phase .
The winter in 2013-2014 across Midwest US was due to negative PNA because there was higher than average atmospheric pressure off the coast of Northern America. Thus option(c) is the correct answer.