Can Riachu learn volt tackle in Pokemon Diamond?

Answer 1

YesBut not directly. You would have to breed a Pichu with volt tackle, then raise it into a Raichu.

To get a volt tackle Pichu, first you would need a light ball (which can be found on wild Pikachu,) and either a Raichu or a Pikachu*. Have that Pokemon hold the light ball, then put it in the daycare with a breedable Pokemon and have it lay an egg. The Pichu in the egg should have Volt tackle.

*I think the Pikachu/Raichu either has to be male, or has to be breeded with a ditto for this to work.