What missing code should be added to this class to ensure that calling fibonacci$(6)$ returns $8?$

class TEST

feature

fibonacci$($n: INTEGER$)$: INTEGER

do

if n $<=1$ then

Result :$=$ n

else

Result :$=\_\_\_--$ missing code here

end

end

end

$2$

a$.$

fibonacci$($n $-1)+$ fibonacci$($n $-3)$

b$.$

fibonacci$($n $-1)+$ fibonacci$($n $-2)$

c$.$

fibonacci$($n $-2)+$ n

d$.$

fibonacci$($n $-1)+$ n