The correct answer is (b).
A year has 365 days, which is divisible by 7, the number of days in a week. This means that every year has 52 weeks and 1 day left over. This extra day can be either a Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday.
If the first day of August is Thursday, then the last day of July must have been a Wednesday. This means that the first week of August has 7 days, with Monday being the first day of the week. The second week of August also has 7 days, with Monday being the eighth day of the month. The third week of August has 6 days, with Monday being the fifteenth day of the month. The fourth week of August has 5 days, with Monday being the twenty-second day of the month.
Therefore, there are 5 Mondays in August.
Option (a) is incorrect because there are 5 Mondays in August, not 4.
Option (c) is incorrect because there are 5 Mondays in August, not 6.
Option (d) is incorrect because there are 5 Mondays in August, not none.