// Transform point B into A's local space.
Vector3 bInAsSpace = a.transform.InverseTransformPoint(b.transform.position);

// Find a position along A's x-axis (y = z = 0 in A's local coordinates)
// that matches B's x-coordinate when viewed from A's frame of reference.
Vector3 cInAsSpace = new Vector3(bInAsSpace.x, 0, 0);

// Transform this position from A's local space back to world space.
Vector3 cInWordSpace = a.transform.TransformPoint(cInAsSpace);