What is the ruling on one who is not qualified to make a judgment when asked about issues that have multiple opinions? With which opinion should he answer? And is it permissible for him in this situation to adopt the madhhab of a specific imam, such as Al-Shafi'i?

If the Mufti knows the most preponderant opinion concerning an issue, he must issue a fatwa accordingly, and it is not permissible for him to issue a fatwa contrary to it, unless the questioner is asking about the madhhab of a specific Imam, in which case he should issue a fatwa according to that Imam's madhhab. However, if the Mufti is unable to preponderate one opinion over another, he should mention the different opinions without preponderating any.