This exercise is in the key of D.  We know this because there are two sharps in the key signature - F sharp and C sharp. 

"Do" is on the note D because the music is in the key of D.

Look over the exercise. 

Then play the recording.  There will be a broken chord playing the Do, Mi, So and top Do pitches followed by the starting note. 

Then, sing along with the recording using the solfege words provided.