Why did Rod Rosenstein appoint Robert Mueller III as a special counsel in May $2017?$

Question $7$ options:

a$)$

to investigate the involvement of Russian operatives in the $2016$ election

b$)$

to discern if Hillary Clinton violated protocol by using a private email server as secretary of state

c$)$

to probe financial decisions made by FBI Director James Comey

d$)$

to examine allegations of sexual harassment by then$-$candidate Trump

e$)$

to investigate nepotism within the Trump administration