Final answer:
The three reindeer's names that begin with the letter 'D' are Dasher, Dancer, and Donner. The letter that always follows 'D' in these words is 'a'. To spell the sound [j] in the words, underline 'edge' in 'Dancer', and 'er' in 'Dasher' and 'Donner'.
Step-by-step explanation:
The three reindeer whose names begin with the letter 'D' are Dasher, Dancer, and Donner.
The letter that always follows 'D' in these words is a.
To spell the sound [j] in the words, the letters that need to be underlined are 'edge' in 'Dancer', and 'er' in 'Dasher' and 'Donner'.