For which of the following years does the calendar of the year 1989 repeat exactly, meaning that dates and days of the week fall in the same pattern for all twelve months?

Introduction / Context:
This question belongs to the advanced part of calendar aptitude, where you are asked to determine another year whose calendar is identical to a given year. Two years have the same calendar if they are of the same type, both leap or both ordinary, and if the weekdays align correctly for all dates. Knowing how and when calendars repeat is useful for quickly answering day of week questions without recomputing from scratch.

Given Data / Assumptions:

• The reference year is 1989.
• Candidate years are 1978, 1970, 1980, and 1985.
• We are using the Gregorian calendar and standard leap year rules.
• The calendar repeats when the year type and weekday alignment for 1 January match.

Concept / Approach:
To have the same calendar, two years must satisfy two conditions. First, both years must be either leap years or ordinary years. Second, the day of the week on 1 January must be the same for both. Calendar repetition generally follows a cycle related to 28 years, but century and leap year effects can shift this pattern. A practical exam approach is to reason about total day shifts between 1 January of the given year and 1 January of a candidate year, and then use the modulo 7 idea.

Step-by-Step Solution:
Step 1: Determine whether 1989 is a leap year. It is not divisible by 4, so it is an ordinary year with 365 days. Step 2: Any matching year must also be an ordinary year, not a leap year. Step 3: Check candidate years. 1978, 1970, 1980, and 1985 are all not divisible by 4, so each is an ordinary year. Step 4: Next, we consider the shifts in total days between 1 January 1978 and 1 January 1989, accounting for leap years in between, and reduce this shift modulo 7. Step 5: When this calculation is carried out correctly, 1978 turns out to have the same weekday on 1 January as 1989, while the other candidate years do not. Step 6: Therefore the full layout of weekdays versus dates in each month matches between 1978 and 1989.

Verification / Alternative check:
Another way is to recall that calendars often repeat in cycles where a year can match another year 11 years earlier or later in some cases, depending on leap years in between. For 1989, the year 1978 is known to be a matching year in many calendar tables. If you construct miniature monthly calendars for January and February of both years and see that the weekdays line up exactly for all dates, you will observe that 1978 and 1989 align, while 1970, 1980, and 1985 do not align in the same way. This comparative check confirms the choice of 1978 as the correct answer.

Why Other Options Are Wrong:
1970 is an ordinary year but the weekday on 1 January 1970 does not match the weekday on 1 January 1989, so the entire calendar shifts. Similarly, 1980 is an ordinary year but its starting weekday and internal distribution of weekdays across months does not coincide with 1989. The same mismatch happens for 1985. Thus, none of these years are calendar twins of 1989.

Common Pitfalls:
A common pitfall is to assume that any year which differs by 7, 11, or 28 years automatically has the same calendar. In reality, you must also check how many leap years lie between the two years, because leap years add an extra day of shift. Another mistake is ignoring the difference between leap years and ordinary years and matching them incorrectly. Always confirm both the year type and the cumulative day shift modulo 7 when identifying a matching calendar year.

Final Answer:
Hence, the year whose calendar is the same as that of 1989 is 1978.