To solve this problem, we will use the Fibonacci sequence formula:

$F(n) = F(n-1) + F(n-2)$

Given:

• $F(7) = 13$
• $F(8) = 21$

We will calculate $F(9)$:

$F(9) = F(8) + F(7) = 21 + 13 = 34$

Thus, the correct answer is:

D. $F(9) = 34$

Would you like further details or explanation? Here are some related questions to consider:

1. What is the formula for calculating the Fibonacci sequence?
2. How would you find $F(10)$ using this sequence?
3. Why is the Fibonacci sequence important in mathematics?
4. Can the Fibonacci sequence be extended backward for negative indices?
5. What are some real-world applications of the Fibonacci sequence?

Tip: Remember that the Fibonacci sequence grows exponentially, which is why it appears in nature and finance!