Translation:They remembered their colleagues.
Keep in mind that reflexive verbs always use forms of "to be" to create the passé composé while regular verbs use "to have". In English this would be "They have recalled their colleagues." or we can also use our regular past form "They recalled their colleagues." You cannot just add the word "by" into their sentence. I personally prefer "remembered" here.
Be careful. You may be thinking of Spanish which has some verbs that use the reflexive for the English passive voice. https://studyspanish.com/grammar/lessons/reflexive2
It actually does happen occasionally in English that the verb will use "to be", for example "I was born." is translated as "Je suis née." It is in passé composé form.
Of course it should be fine, because "to remember something" and "to remember about something" are more or less synonymous, for speakers of English. DL in this instance, as so often, is dealing more with people who've learned English than with people who know English natively. (In fact, as a native English speaker who's trying to learn French, I find it useful to connect "se souvenir de" with "to remember about," in part to help me remember the needed preposition in French.)
Yes, you can remember something about someone, but that would be a different sentence in French.
Je me souviens de quelque chose à propos de quelqu'un.
I remember something about someone.
If your entire answer was exactly like their correct answer, then report it please. Keep in mind that the error could be somewhere before that word even if it is highlighted. For example, "leurs" must also be plural if "collègues" is plural. Did you put "souvenues" in plural to agree with the subject?
Well you can tell that the subject "elles" is plural from the verb form "sont" and the past participle which comes after the "to be" form must agree with the subject "souvenues", but you cannot tell whether the last word is plural or not and both should be accepted if you have the listen and write it down in French. Keep in mind that the possessive adjective "leur" or "leurs" must agree with the number of the word that is possessed. So you should be able to use "leur collègue" or "leurs collègues".
However for any other exercise, you have a visual to see what is plural and what is not.
Reflexive verbs always use "to be" instead of "to have" in French. Just something to memorize. Here is a site that also has a list of other verbs that require être instead of avoir: https://www.frenchtoday.com/blog/etre-versus-avoir-french-auxiliary-verbs-past-tenses
ElaineH - I know what you mean. In this case where there is no difference in pronunciation between singular and plural, both should be accepted. And this was noted to DL YEARS AGO. Last night I came across a completely mispronounced word on the recording that people have been complaining about for FOUR YEARS and it's still there. Makes you wonder if anybody's home.