Two identical capacitors are joined in parallel, charged to a potential V and then separated and connected in series, i.e., negative plate of first is connected to positive plate of second:
Answer & explanation
Correct answer: option 3
When they are connected in parallel, each capacitor will have a potential V. When they are joined in series having each of potential V, then total potential across the two extreme plates of the capacitor is
From Charge conservation $CV + CV = 2CV'$
$V'= 2V$