Final answer:
In the CRISPR-Cas system, tracrRNA binds to crRNA and Cas9, creating a complex that guides the Cas9 protein to cleave phage DNA.
Step-by-step explanation:
In the CRISPR-Cas system, the tracrRNA binds to crRNA and Cas9. During the immune response of certain bacteria and archaea, the system engages in a process to defend against phage infections. When a phage infects a prokaryote, spacer DNA from previous infections is transcribed along with CRISPR sequences and Cas genes. The resulting CRISPR/spacer RNA then forms hydrogen bonds with a complementary region of the tracrRNA, and this complex associates with the Cas proteins, such as Cas9. The Cas9 endonuclease is guided by the resulting RNA duplex to the matching DNA sequence in the phage genome, where it cleaves the DNA, thereby providing immunity against the phage.
Therefore, the correct answer to the question 'In the CRISPR-Cas system, what does the tracrRNA bind to?' is C. crRNA and Cas9.