The blank is correctly filled with: “a federally approved child passenger restraint system.”

In context, many multiple-choice versions of this question list options like:

  • A. A federally approved child passenger restraint system
  • B. Booster seat or safety belt when transported in any motor vehicle other than a public carrier for hire or school bus
  • C. All of the above

The keyed correct answer given for this exact question is “a federally approved child passenger restraint system.”