Is optimumnutrition seriousmass gainer approved by fda..??is it safe to take..??

Unsure: WIthout seeing the full ingredients it is hard to say, but most of these are laden with whey protein. As for FDA, most supplements I believe are not under the jurisdiction of the FDA. I would tell you to just eat right and exercise. If you are healthy otherwise and are looking to add muscle weight, some will say to have 1 gram of protein for every kilogram of body weight you are daily.