The line that best demonstrates Tarzan's reason for helping Mr. Philander and Professor Porter is:
A) "It was Tarzan of the Apes who watched, with face a-grin, this odd game of follow-the-leader."
This line indicates Tarzan's amusement and engagement with the situation, suggesting that he is observing the men and their predicament, which reflects his interest in their well-being and his inclination to assist them.