The probability of a non-leap year having 53 Mondays is:

1. $\Big(\dfrac{1}{7}\Big)$

2. $\Big(\dfrac{2}{7}\Big)$

3. $\Big(\dfrac{5}{7}\Big)$

4. $\Big(\dfrac{6}{7}\Big)$

In a non-leap year (an ordinary year), there are 365 days.

365 days = 52 weeks + 1 day

This 1 extra day can be any of the following 7 possibilities: {Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday}

Total number of possible outcomes = 7

Number of favourable outcomes (The extra day being a Monday) = 1 (i.e., {Monday}).

Let E be the event that a non-leap year has 53 Mondays,

∴ P(E) = $\dfrac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}} = \dfrac{1}{7}$

Hence, option 1 is the correct option.