does the mass of a body decrease or increase with its velocity reaching the velocity of light?

Well...mass is constant for a body a ordinary speeds; however, when the speed of body is comparable with the speed of light 'c', the mass increases with the increase in speed and tends to infinity when velocity tends to 'c' in accordance with the relation :-

m = m_o = [ 1 - ( v² / c²) ] ^1/2.