(N/A) $(i)$ Variable resistor or rheostat: It is used to change the resistance in a circuit,thereby controlling the current flowing through it.
$(ii)$ Wires crossing without joining: It represents two wires that cross each other without making any electrical contact.
$(b)$ The circuit diagram consists of a $12 \, V$ battery,a switch $(K)$,an ammeter $(A)$ in series with the combination,and a voltmeter $(V)$ connected in parallel across the three resistors ($5 \, \Omega$,$10 \, \Omega$,and $20 \, \Omega$) which are themselves connected in parallel.
$(c)$ Advantage of parallel connection: In a parallel circuit,if one electrical device stops working due to a defect or switch-off,the other devices continue to operate independently because each device has its own separate circuit path.