To answer this question, we need to analyze the given statements and determine the validity of the third statement.
Statement 1: All the tulips in Zoe's garden are white.
Statement 2: All the pansies in Zoe's garden are yellow.
Based on these two statements, we can conclude that there are only white tulips and yellow pansies in Zoe's garden. However, the third statement claims that "all the flowers in Zoe's garden are either white or yellow."
Since the third statement is a generalization that includes all flowers in Zoe's garden, we cannot be certain if there are any other types of flowers apart from white tulips and yellow pansies. It is possible that there may be other types of flowers in different colors.
Therefore, the correct answer is C. uncertain.