"Selection risk - since scheme's standard benefits only provide a pension for member, the take-up of the option to provide dependant's benefits is likely to be quite high - this reduces need to adjust assumptions to reflect risk of selection." - Why does this reduce the need to adjust assumptions to reflect the risk of selection?
Selection is essentially the case where those who benefit more (than average) take the options and those who benefit less (than average) don't. To allow for selection means adjusting the assumptions to suit those taking the option rather than the average for the population. If almost everyone takes the option however, there really isn't much selection. the experience of those who "selected" is the same as the population as a whole.
