If you know the expected value is 500 and the standard deviation is 12, you can say with approximately a 95 percent confidence rating which of the following?

The activity will take from 488 to 512 days.

The activity will take from 464 to 536 days.

The activity will take from 494 to 506 days.

The activity will take from 476 to 524 days.

 

The answer is D. But my question is how did they arrive at this answer. Kindly show calculation steps?