First of all, I am unclear as to whether the answer to this question is a matter of fact or of opinion. Nevertheless, I hope to find the answer.
For reference: a normal P QRS from an EKG from https://www.healio.com/cardiology/learn-the-heart/ecg-review/ecg-interpretation-tutorial/qrs-complex
The question and the different looking beat: one cannot say it is ectopic because the strip represents some sort of atrial arrhythmia. Atrial fibrillation is the most common of these. The beats are not regularly spaced and there is no sign of a P wave (the one produced by the atrium).
Atrial Fibrillation occurs when multiple electrical impulses occur within within the atria. This chaotic electrical activity results in a chaotic wave form between the QRS complexes. P waves are absent. They are replaced by lower case "f" waves. No P waves means there is no PR interval measurement. This rapid electrical activity overwhelms the AV node causing impulses to enter the ventricular conduction system at irregular points. This results in irregular R to R intervals.
"Ectopic" beat suggests there is a regular beat going on and some rogue beat elbows its way in. In A fib they are all rogue beats - "multiple electrical impulses". The one you see is not in structure different from the rest - there is a T for every QRS and so they are all coming from the atrium. It is just that with that beat the S wave is deeper. Maybe because it is so close to the T? I bet if you ran that strip a while you would see others comparably deep.
I am betting this is homework. What exactly is the question?