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Frieda the frog begins a sequence of hops on a $3 \times 3$ grid of squares, moving one square on each hop and choosing at random the direction of each hop-up, down, left, or right. She does not hop diagonally. When the direction of a hop would take Frieda off the grid, she "wraps around" and jumps to the opposite edge. For example if Frieda begins in the center square and makes two hops "up", the first hop would place her in the top row middle square, and the second hop would cause Frieda to jump to the opposite edge, landing in the bottom row middle square. Suppose Frieda starts from the center square, makes at most four hops at random, and stops hopping if she lands on a corner square. What is the probability that she reaches a corner square on one of the four hops? 
Overseas Competition->Knowledge Point->Counting Modules
We will use complementary counting. First, the frog can go left with probability $\frac{1}{4}$. We observe symmetry, so our final answer will be multiplied by $4$ for the $4$ directions, and since $4 \cdot \frac{1}{4}=1$, we will ignore the leading probability. From the left, she either goes left to another edge $\left(\frac{1}{4}\right)$ or back to the center $\left(\frac{1}{4}\right)$. Time for some casework.  Case 1: She goes back to the center. Now, she can go in any $4$ directions, and then has $2$ options from that edge. This gives $\frac{1}{2}$.  Case 2: She goes to another edge (rightmost).  Subcase 1: She goes back to the left edge. She now has $2$ places to go, giving $\frac{1}{2}$  Subcase 2: She goes to the center. Now any move works. $\frac{1}{4} \cdot \frac{1}{2}+\frac{1}{4} \cdot 1=\frac{1}{8}+\frac{1}{4}=\frac{3}{8}$ for this case.  She goes back to the center in Case 1 with probability $\frac{1}{4}$, and to the right edge with probability $\frac{1}{4}$ So, our answer is $\frac{1}{4} \cdot \frac{1}{2}+\frac{1}{4} \cdot \frac{3}{8}=\frac{1}{4}\left(\frac{1}{2}+\frac{3}{8}\right)=\frac{1}{4} \cdot \frac{7}{8}=\frac{7}{32}$ But, don\textquotesingle t forget complementary counting. So, we get $1-\frac{7}{32}=\frac{25}{32} \Longrightarrow D$. 