Answered below

Thank you 

Bonjour de nouveau Wendy,

I have revisited this query of yours and I can see now what the issue is with using "on" as the subject pronoun.

What happens is that when "on" is employed as the third person subject pronoun the past participle concurs with what "on" is. The auxiliary remains third person singular, only the past participle changes with the subject number(s)

So when "on" represents two females and être is the auxiliary, the past participle concurs with two female subjects.

"Quand on s'était arrêtées ......"   The second "e" female and the "s" plural so we have PP ending ...ées   (The past participle is taking the role of an adjective - and relates to the subjects)

However with avoir as auxiliary then there is no concordance required "On avait croisé...."

Bonne Journée

Jim

I think that’s what I did: the subjects were two females, the verb reflexive.

Hi Wendy,

s'était arrêtées is correct.    How do I know?     I got it wrong and this is the correct answer. I did the quiz earlier this afternoon and checked my errors.

Jim

Thank you everyone