How should a pilot respond verbally to being cleared for takeoff?
This is how FAA recommends (PDF):


  A typical takeoff clearance may state, for example, “(Callsign) 123
  RNAV to MPASS, Runway 26L, Cleared for Takeoff”.
  
  The expected pilot response is, “(Callsign) 123, RNAV to MPASS, Runway
  26L, Cleared for Takeoff”.