On 'a' Thursday does mean 'every' Thursday. In general, 'a/an' can mean 'any' or 'all/every' of something. I wouldn't like to bump into a lion. [any lion] An elephant is bigger than a squirrel. [any or every elephant] Elephants are bigger than squirrels. I like an apple. I like apples. So, a 'something' is often the same as 'somethings'. on a Thursday = every Thursday = on Thursdays

Hey Lucy, that's a very good question! Katie is referring to a weekly schedule where each day of the week happens once. She is indicating that the session happens once within the context of a week. Just once a week : she clarifies there in one training session per week On a thursday: she highlights that this singular training session happens on thursdays. If she hadn't mentioned 'just once a week' before mentioning Thursday, she would have said 'on thursdays'.

You are correct the audio could have said . "on Thursday evenings" or "Every Thursday" . Both are the same.But the test and questions are it seems almost obviously listening questions. Not grammar questions.

Both are correct.

I would say "Just on Thursdays". "Once a week" is superfluous since Thursdays only happen once a week. If I wanted to insert "once a week", I would say "Just once a week - Thursdays", skipping the prepositions entirely. However, all of your suggestions and also the comments are good alternatives.