Is it ok to take alka seltzer plus cold and flu and take clartin d in the same day?

Multiple medications. Before you start treating yourself, you must have a diagnosis in mind. Do you think you have the flu? If so, none of the items you are taking offer any effective treatment for it. They are a little bit like chicken soup, they may not help you and they cannot hurt you. If you have phlegm that is greenish in color, see a physician for antibiotics. Good luck.
Mixing cold meds. It may be safe since most otc medications have a pretty good safety margin but you may be taking double doses of decongestants. Mixing meds can be dangerous. The best bet is to ask your pharmacist. Many of the medications have multiple versions with different ingredients. It is especially dangerous to mix medications that have Acetaminophen or tylenol (acetaminophen) in them.