9/8 per hour
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Explanation:
Unit rate defined as the rates are expressed as a quantity 1 such as 4 meter per seconds or 4 miles per hour.
As per the statement:
Kelly is knitting a scarf for her brother. it took her 1/3 hour to knit 3/8 foot of the scarf.
She took /3 hour to knit 3/8 foot of the scarf. Therefore, 9/8 feet per hour fast is Kelly's knitting speed.