The passage says:
1. Multicultural education aims to give objective, unbiased knowledge of other cultures.
2. But a practical problem is that there's not enough time in the school year to cover each individual nationality equally.
3. Solutions:
- Make school year longer → unlikely due to political aspects.
- Modify curriculum to include only what the school/instructor thinks is most important → but then they're open to criticism for not treating all groups equally.

Now, the question asks: Which is a problem associated with introducing a truly multicultural education system in modern schools?
- A - "The additional time needed to teach all possible aspects of a subject" → Yes, the passage says there's not enough time to equally cover all nationalities' contributions.
- B - "The unwillingness on the part of political leaders to go ahead with such an education system" → Not exactly. The passage says political aspects make extending the school year unlikely, not that political leaders oppose multicultural education itself.
- C - "The subjectivity that may creep in if the school were to decide which aspects to teach and which to skip" → Yes, the passage says if they modify curriculum to include only what they think is important, they're open to criticism - that's about subjectivity in selection.

So $\(\mathbf{A}\)$ and $\(\mathbf{C}\)$ are supported.