Why is the spirit in the second stave holding a branch of holly?
Holly is very symbolic of the Christmas season, so the Spirit of Christmas Present would naturally have a sprig of holly.
"At the time when Dickens was writing, the Christmas tradition was not nearly as important as it is today. Celebrating Christmas started in the fourth century, incorporating many of the symbols, such as holly and wreaths, of pagan holidays, such as the Roman Saturnalia and the Saxon Yule holiday."
The Ghost of Christmas Present brings Scrooge into the middle of the present day event. He is given an opportunity to watch the hustle and bustle associated with the holiday. But, the story is also one of redemption in the Christian sense. Therefore, holly also has a symbolic Christian meaning, its pointed leaves and red berries are representative of Christ's crown of thorns.
Since it is urgent that the spirits show Scrooge the goodness of mankind as well as the suffering it is logical to bring the Christian message into the meaning. Remember this spirit tells Scrooge that mankind was his business.
The spirits are trying to convert Scrooge's heart to be loving and compassionate.