to define double squared multi dimentional array of size nxn a$.$double $[,,]$a$=$new double$[$n$,$n$]$;

b$.$

double $[][]$a$=$new double$[$n$]$;

c$.$

double $[,]$a$=$new double$[$n$,$n$]$;

d$.$

double $[,]$a$=$new double$[$n$][]$;