A simile is a simple, direct comparison between two things, such as "The snow is a white blanket" or "Life is a rollercoaster". A metaphor is a more indirect or complex comparison. For example, "The snow blanketed the hill, as if to keep the sleeping land warm" is a metaphor. The snow is again being compared to a blanket, and the land to some being that could sleep, arguably a form of personification. "The fog crept in on little cat feet" is both a metaphor and a personification. The fog's motion is being compared to that of a cat, and the fog itself is being represented as a living being, a cat. (Note that personification need not involve a human.)
"He dived from the board, his body a spear as it entered the water" is metaphor, and may or may not involve symbolism, depending on what connections the rest of the text may make between the diver and a spear.
"The trees start whispering among themselves" is a personification, but it also acts as a metaphor, comparing the sounds made by the trees, or perhaps by the wind, to whispers. Indeed most personifications are also metaphors. But since "personification" is more specific than "metaphor" it would be the correct answer in a class setting, most likely.