When 6.3 g sodium bicarbonate and 30.0 g acetic acid are mixed together, the total mass must be 36.3 g. If this mass decreased to 33.0 g, it must be due to the release of CO2. Therefore, the mass of CO2 released = 36.3 g - 33.0 g = 3.3 g. So, the answer choice is 4.
Incidentally, it is not correct to use gm for gram. The correct abbreviation is g.