Which of the following Java code snippets correctly checks if a binary tree is a binary search tree?

a$.$

boolean ISFun $($Node node, int min$)$ i

int max$=$ size $-1$ ;

if $($node $==$ null$)$

return true;

if $($node$.$ key $$ min & s node. key $>$ max$)$

return false;

return ISFun $($node; node. left, node. key$-1)$ & &

ISFun $($node: node. right, node. key$+1)$ ;

b$.$ boolean ISFun $($Node node, int min$)$ f

int max$=$ size $-1$ ;

if $($ node $==$ null$)$

return true;

if $($node$.$ key $$ min $||$ node. key $>$max $)$

return false;

return ISFun $($node: node. Ieft, min: node.key$)$ &&

ISFun $($node: node. right, mini node. key$)$;

c$.$ boolean ISFun$($Node node, int min, int max$)($ if $($node $==$ nu$11)$

return true;

if $($node$.$ key $$ min $1$ node, key $>$ max$)$

return false;

return ISFun$($node: node. left, min, node.key$-1)$ &&

ISFun $($node: node. right, node. key$+1,$ max $)$;

d$.$ boolean ISFun $($Node node$)1$

if $($node $==$ nu$11)$

return true;

if $($node$.$ left $$ null & &

node. left. key $>$ node. key$)$

return false;

if $($node$,$ right $I=$ null & &

node. right. key $$ node, key$)$

return false;

return ISFun $($node: node, left$)$ &&

ISFun $($node; node. right$)$ ;