In this question PC and GPR are carried out in the ALU.
So, PC and GPR performing write operation.
"call Rn, sub” - here subroutine call is performing , which is a memory read operation
And each memory read operation takes 2 clock cycle.
Then operation is performed in ALU.
$1)PC_{out},MAR_{in},Read,IncPC$
$2)WMFC$
$3)R_{2out},R_{3out},SelectA,SUB,R_{1in},END$
First two are FETCH cycle and 3rd one executing SUB instruction,i.e. EXECUTE cycle
Total 3 clock cycle