Eating one of what fruit a day is said to keep the doctor away?

"An apple a day keeps the doctor away" is a common English-language proverb that appeared in the 19th century, advocating for the consumption of apples, and by extension, "if one eats healthful foods, one will remain in good health and will not need to see the doctor often." A variant of the proverb, "Eat an apple on going to bed, and you'll keep the doctor from earning his bread" was recorded as a Pembrokeshire saying in 1866. The modern phrasing, "An apple a day keeps the doctor away", began usage at the end of the 19th century, with early print examples found as early as 1887.

Source: Wikipedia