Which of the following stopping sounds will take the longest for a child to cease as a part of normal development?

The correct choice is the stopping of voiceless "th" sounds, as it is common for young children to replace certain sounds with others as part of typical speech development. In this context, voiceless "th," represented by the sound as in "think," often tends to be one of the last sounds that children acquire or fully master. This is because the voiceless "th" sound has no direct equivalent in many languages, making its development trickier for children.

A child's progression in pronouncing challenging consonants typically shows that stops like "t" or "s" are acquired more easily and earlier. The voiceless "th" sound requires a specific tongue placement and airflow that can be particularly difficult, and thus, it is common for children to substitute it with easier sounds, such as "t" or "f," for a longer period.

In contrast, the voiced "th" (as in "this") and other sounds like "s" and "z" are integrated into a child’s speech pattern more readily. Consequently, the substitution of voiceless "th" usually remains present for a longer timeframe until the child acquires full command of these more complex sounds.