When water receives an electron from a metal, the water molecule becomes unstable. it splits up into a neutral hydrogen atom and a oh- ion. the neutral hydrogen atoms, each with one electron, randomly wander around until they find each other. this random wandering around is referred to as

The reaction will be like

Now these H will wonder around means they will diffuse here and there (the random transnational movement of particles is known as diffusion) and once the two hydrogen atoms come across each other they will form bond and will form hydrogen molecule

so answer is : diffusion

lol for the pointsss