Lanthanoids are

• A
  $14$ elements in the sixth period (atomic no. $= 58$ to $71$) that are filling $4f$ sublevel
• B
  $14$ elements in the seventh period (atomic no. $= 58$ to $71$) that are filling $4f$ sublevel
• C
  $14$ elements in the sixth period (atomic no. $= 90$ to $103$) that are filling $4f$ sublevel
• D
  $14$ elements in the seventh period (atomic no. $= 90$ to $103$) that are filling $4f$ sublevel