The note D is written on the fourth line of the staff.
Play the note D with the third finger of your left hand as shown in the photo. The space between the second and third fingers when playing C# and D is less than the space between the first and second fingers when playing B and C#.
Don’t worry if you have trouble with finger positions at first. With a bit of practice, your fingers will naturally go to the correct position on the fingerboard. Remember to practice your songs both pizzicato and arco.