Final answer:
For a 35-day menstrual cycle, the couple should have intercourse around the 17th to 21st day to increase their chances of conception, making the 21st day a likely time.
Step-by-step explanation:
The optimal timing of intercourse for a couple trying to conceive when the woman has a 35-day menstrual cycle would not be the 14th day as it is for the average 28-day cycle. Since ovulation typically occurs around the midpoint of the cycle, for a 35-day cycle, ovulation would more likely happen around day 17 to 21.
Therefore, the couple should aim to have intercourse a few days before and up to this point to increase the probability of conception. The statement that they should have intercourse on the 21st day of the cycle would be the most likely correct answer based on the information provided.