I forgot to offer one Rak`ah (unit of Prayer) in the Zhuhr (Noon) Prayer and only remembered after offering the Maghrib (Sunset) Prayer? What should I do?
You are obligated to repeat the Zhuhr Prayer because one Rak`ah was missed and a long period of time elapsed between missing the Rak`ah and remembering it. If the time between the two actions is not long, you should offer the missed Rak`ah and perform Sujud-ul-Sahw (Prostration of Forgetfulness). It is better in this case, and others similar, to perform Sujud-ul-Sahw after Taslim (salutation of peace ending the Prayer).