What command find all occurrences of the strings mail or Mail in the file

$/$var$/$tmp$/$error$.$log$?$

a$.$ grep $\backslash [$mM$\backslash ]$ail $/$var$/$tmp$/$error$.$log

b$.$ grep $\backslash [$Mm$\backslash ]$ail $/$var$/$tmp$/$error$.$log

c$.$ grep $[$mM$]$ail$/$var$/$tmp$/$error$.$log

d$.$ grep $[$mM$]$ail$/$var$/$tmp$/$error$.$log

e$.$ grep $[$mM$]$ail $/$var$/$tmp$/$error$.$log

f$.$ grep $[$mM$]$ail $/$var$/$tmp$/$error$.$log

g$.$ all of the above