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 Do Re Mi (solfege) words provided. This will be sung an octave lower than what is written.