The vulnerability lies in line [10], where the function strcpy(transmit, input) is used. The strcpy function does not perform boundary checking when copying strings. Since input is defined with a size of 256 characters and transmit only has 20 characters allocated, the strcpy operation will cause a buffer overflow when the contents of input exceed the allocated size of transmit. This creates a significant security vulnerability, as attackers can overwrite adjacent memory, potentially injecting malicious code or altering program execution.

Lines [02], [04], [07], and [08] are not inherently vulnerable by themselves. Line [04] defines the oversized input, but the vulnerability only materializes when combined with the unsafe copy in line [10]. Secure coding practices recommend using safer alternatives like strncpy, which includes a length parameter, or implementing runtime checks to ensure the destination buffer size is not exceeded.

Thus, the vulnerable line that must be changed is line [10], where strcpy is used.