Option D is correct because it properly retrieves the private key from the environment variable, decodes the PEM format, and passes it to the connection parameters after converting the key object to a string. It uses the cryptography library to handle the private key securely, which is required for key pair authentication. The code snippet decodes the private key using cryptography library and creates a string, which then is passed to the connection parameters to create a session. Option A fails to use the cryptography library and just passes the private key directly. Option B is incorrect because it doesn't use the 'configs' method, which is the correct way to pass all connection parameters at once. Option C doesn't encode the private key properly, and does not convert to string after decoding using cryptography. Option E does convert to string, but is not secure as the private key is directly passed without decoding it using the cryptography library.