If you choose any of the three playing cards of "hearts 5, hearts 7 and hearts 9", the probability of winning "hearts 7" is a.b.c.d 。

Analysis: According to the meaning of the question, there are 3 playing cards, of which 1 is "7 of hearts". According to the calculation formula of probability, the answer can be obtained.

According to the meaning of the question, there are three playing cards, of which 1 is "7 of hearts".

Then the probability of drawing "7 of hearts" is,

So choose B.