Final answer:
The diagnosis is most likely Atrial Fibrillation, which is identifiable on an ECG by an abnormal electrical pattern prior to the QRS complexes and a higher frequency of QRS complexes.
Step-by-step explanation:
Based on the information provided that notes an abnormal electrical pattern prior to the QRS complexes and increased frequency between the QRS complexes, the most likely diagnosis is (B) Atrial Fibrillation. Atrial fibrillation is characterized by rapid, irregular electrical impulses in the atria leading to a disorganized and fast heartbeat. This can be seen on an ECG as an irregular baseline with varying intervals between QRS complexes and absence of distinct P waves.