My back tooth molar is decayed what is best extraction or fill in ?

Keep if you Can: It's always best to keep natural teeth if you can. Nothing functions as well as the original equipment. That said, if the costs are more than you can manage or you have multiple needs and have to make choices, or the decay is excessive & the tooth cannot be repaired, there are multiple solutions for replacing a missing tooth. Your dentist should be able to guide you & help you make the best choice.

Listen to Dentist: Your dentist should be able to give you recommendations to help you decide what is best for you. If it is fixable with a high success rate, fix it. If not then maybe you will need it removed. If you take it out, replace it. An implant is a great option but there are others that may work for you as well. All teeth help function and are important.

Try to save: It's almost always best to save teeth if possible. Nothing functions as well as natural healthy teeth. I often give this analogy to patients: "if you had severe arthritis of your fingers, would you want them surgically removed?" what about your toes? There are reasons when it becomes better to extract a tooth. Ask your dentist to explain to you what would be best for your and why. Then decide.