Translation:I betrayed my colleagues and secretary.
Can 'các' extend its scope to the secretar[y|ies] here? If it can, I assume this is optional?
Yep. If there was only 1 secretary then it'd be "các đồng nghiệp và một người thư ký"
OK, so there is mandatory plural interpretation on 'secretaries' here, meaning the given (singular) answer is actually inaccurate?
It's an ambiguous sentence because it can mean both "my colleagues and secretaries" or "my colleagues and secretary".
I feel like it would be fine if they had the singular first. That or they would need 2 sentences