The correct option is D Around day 20
The duration of menstrual cycle differs from person to person. In the same person, it might differ again from cycle to cycle. The duration of 28 days is considered ideal and is achievable by only a few women. Majority of the females have a duration either greater than or lesser than this.
Ovulation occurs 14 days prior to the next menstrual cycle. So, if the duration of a cycle in a menstruating female is 35 days, the ovulation is supposed to occur around (35 - 14) day 21. Considering the day of ovulation to be day 21, LH surge occurs one day prior to it, i.e. day 20.
In an ideal menstrual cycle of 28 days, ovulation would occur around (28 - 14) day 14. LH surge will occur the previous day, i.e. day 13.