In the given piece of DNA, The total number of bases adenine, thymine, cytosine and guanine is $3.2 Kbp$ which is equal to 3200 base pairs.
i.e., $A+T+C+G=3200$
or, $(A+T)+(C+G)=3200$
or, $2 A +2 C =3200$ since bp of $A = T$ and $C = G$
or, $A+C=1600$
or, $C=1600-A=1600-820$
or, $C =780$
So, the number of cytosines will be 780 bps.
Explanation of the incorrect option:
Option B: the number of cytosines will be $780 bps$.
Option C: the number of cytosines will be 780 bps.
Option D: the number of cytosines will be 780 bps.
Final answer: In a $3.2 Kbp$ long piece of DNA, 820 adenine bases were found.
The number of cytosine bases is 780 .