The statement that is an example of a beneficial mutation is:
"A mosquito has a mutation that causes it to be more resistant."
This type of mutation can be advantageous because increased resistance can help the mosquito survive in environments with insecticides or diseases, allowing it to thrive and reproduce. In contrast, the other examples either do not confer a clear benefit (e.g., white spur color, attached earlobes) or represent a condition that can lead to difficulties (e.g., lactose intolerance).