안녕하세요 이연욱 샘입니다.

개념 설명하는 강의에서 설명했습니다. 간단히 정리하면,

To perform a significance test for the slope of a regression line, you must check 5 conditions:

1. SRS

2. Normality

3. Independencej

4. Linear

5. Equal variance

In given residual plot, residuals are getting bigger as x increases which tells me that 5th condition(equal variance) is not met. Therefore (C) is the correct answer.