A. 7^j mod 19
B. 1 mod 19
C. 7^3 + 7^j mod 19
D. All of the mentioned are true
I had been asked this question during a job interview.
This is a very interesting question from Number Theory topic in division More Number Theory of Cryptograph & Network Security
To explain I would say: 7^(3+j) mod 19 = 7^3 * 7^j mod 19 = 7^j mod 19 ( since 7^3 mod 19 = 1 ).