A 55-year-old female is evaluated in the hospital for a single 10-minute episode of chest pain at restwhich occurred 1 hour prior to presentation. An EKG shows 1 mm ST segment depressions in leads V4-V6. What is the most likely diagnosis? 
a) Stable angina 
b) Unstable angina 
c) Acute myocardial infarction (NSTEMI) 
d) Prinzmetal's angina (variant angina)