Hello Sir! This is Tejas patange. I have studied the lesson of the projectile motion. I have learnt that "the horizontal range of projectile is the same for any two angles, (thita & 90-thita)(projected with same velocity)". My question is that does this term remains valid for the angles-0 & 90 degrees??

Yes, it's valid for theta = 0 and for theta = 90 degrees, assuming that the launching point is at height = 0. In both cases the range is 0. For the case of theta = 90 the projectile goes vertically straight up, and then falls straight down back to where it started. For the case of theta = 0 degrees the projectile comes out horizontally and immediately hits the ground - since it never had any altitude it spends zero time in flight.