The poor lady had to go out for food ______ a cold morning

The poor lady had to go out for food ______ a cold morning

正确：on (特指某天的早上、中午、下午晚上时,常使用介词on)

on
只要有morning就用介词on

on +某一天

on，morning前有cold这个修饰词，表示特定的、指定的早晨，像这种具体的用on，不用in。