The suggested correct text is בחודש שעבר, but try as I might, I can hear not the faintest trace of an opening ב ... Clipped audio ? Small typo or other glitch ?
Kind of depends on the context. It could be simply בחודש שעבר, like here, or במהלך החודש שעבר, the latter being like "over the course of".
The "official" correct answer was just given in an exercise as "In last month" which isn't good English, unlike the translation shown above.