Answer:
A shift in the central ideas of a sonnet called: "a volta"
The statement that best explains the shift in the central ideas of "Sonnet 91" is: "The speaker realizes that love might be taken away."
Explanation:
A Volta is a shift in the thought in a sonnet. When the shift is about to occur, initial words like "But, Yet, And..." are seen in the lines.
"The speaker realizes that love might be taken away." is the statement that best explains a shift in the sonnet. This is seen when the speaker said: "Wretched in this alone, that thou mayst take All these away...".