Best place to place a decoupling capacitor
Think of the copper tracks as series inductors. Series inductors are bad, you want them as small as possible. (B) is the better option.

Also loops in your tracks are bad, again they form an inductor and easily pick up (or radiate) an EM-field. You want the surface area of loops as small as possible, thus keep forward and return paths as close to each other as possible. (C) is the better option.