Determining data sufficiency for calculating a combined work rate

To find when a calendar repeats, track the accumulation of odd days (remainders modulo 7). A non-leap year has 1 odd day, while a leap year has 2 odd days. A calendar repeats when the total accumulated odd days hit a multiple of 7 and the target year matches the original year's type (ordinary vs. leap).

2025 is an ordinary year. Track from 2025 onwards:

The sum reaches 7 after 2030, meaning the next year, 2031, begins on the exact same day of the week[cite: 1640, 1641]. Since 2031 is also an ordinary year, the entire calendar matches perfectly.

Answer: (c).