6. Four people, A, B, C, and D, are playing a passing game. In the first pass, A passes the ball to one of the other three people. In the second pass, the person who receives the ball passes it to one of the other three people. This passing continues for a total of 4 times. What is the probability that the ball is passed back to A on the fourth pass?
(A) $\frac{7}{27}$
(B) $\frac{5}{27}$
(C) $\frac{7}{8}$
(D) $\frac{21}{64}$
6.A.

Draw the tree diagram as shown in Figure 5.
From Figure 5, we know that the third pass cannot be made to A. There are 6 times when A can receive the ball, so there are $27-6=21$ times that meet the condition. The number of ways for 4 passes is $3^{4}$, thus the probability that meets the condition is $\frac{21}{3^{4}}=\frac{7}{27}$.