By default all model statistics are calculated across the whole population. Hence the answer is (b).
If you want (a), you can use simoptions.conditionalrestrictions to restrict the population on which the statistics are calculated to be those who are participants. [The restriction should be set up to =1 for participants, 0 otherwise.]
Note, the above relates to ‘AllStats’. With ‘AgeConditionalStats’ everything is the same as above, but conditional on age. So the basic would be “risky share across both participants and non-participants, conditional on age”, while you could use simoptions.conditionalrestrictions to get the “risky share across participants, conditional on age”.