Its actually easy. Its like u r in a boat, an accident occurred... and it turned over or something. Now u have urself, ur family and ur mom, who are u gonna save? Some went for their wives, others for their kids, some didn't tried 'cos according to them their own lives are important...and....Muslims for their moms. Explanation, simple... wives and kids are replaceable... Mothers... NOT.
Though you have mentioned 'Muslims', I dont believe this trick question was meant to highlight how much muslims revere there mothers. I think that this question (and the answer to it) is strictly numerical in nature, specially, the way you have put it forward, with wives and kids being technically replaceable, since you can always remarry, and can have more kids, but only one woman (your mon) will ever give birth to you.
You guys dont crucify Cold Fire for this statement. I dont think it was in any way meant to state that wives and kids are (god forbid) dispensable. It was just a numerical answer to what seems to be a trick question. Otherwise, if you want to dissect this question from an emotional perspective, no sane person will ever be able to make a decision and live with it.