Final answer:
Ben Franklin was referring to asparagus, which contains a substance that gives urine a strong odor after consumption.
Step-by-step explanation:
The vegetable Ben Franklin was referring to when he noted that eating a few stems of it will give our urine a disagreeable odour was asparagus. This observation is likely tied to Franklin's broad interests which spanned the natural sciences as well as his role as a statesman and inventor. Asparagus contains asparagusic acid which, when digested, produces sulfur-containing compounds. These compounds give urine a characteristic strong odor which is quickly noticed by those who have consumed this vegetable.