Have you ever left your house, driven afew miles down the road and had this wierd feeling that you forgot something? Well, why is your brain smart enough to be aware that something has been forgotten, yet completely unable to know exactly what it is that was forgotten, until its way too late.
Irishman said
In an ideal world, yes, IQ tests would measure intelligence directly. But as a practical matter, test makers resign themselves to measuring the products of intelligence, including vocabulary. One of the reasons for this is that, out of all the subtests on an IQ test, your score on vocabulary is almost always the best estimate of your full-scale IQ. Statistically, the vocabulary subtest has the highest loading on g, the general factor of intelligence. For this reason, all of the most commonly used IQ tests have a vocabulary section.
This is true for “fluid” intelligence, which used to be what psychologists meant when they talked about intelligence. But in recent years, psychologists have recognized that “crystallized” intelligence, such as vocabulary and your general knowledge about the world, was a legitimate form of intelligence in its own right. So today, many IQ tests have a verbal scale, which measures crystallized intelligence (more or less), and some type of non-verbal scale for measuring fluid intelligence.
As to the OP, the answer is obviously TQNK, since that is the only group that doesn’t contain any letter with two planes of symmetry. 
If anyone is interested, the same problem was given in this article in the NY Times. (Free registration may be required to view it.) The source is the journal Science. The answer is also given, namely TQNK, and the reason given is the same as gigi’s: “the letters are equally spaced in the alphabet.”
quote:
But in recent years, psychologists have recognized that “crystallized” intelligence, such as vocabulary and your general knowledge about the world, was a legitimate form of intelligence in its own right. So today, many IQ tests have a verbal scale, which measures crystallized intelligence (more or less), and some type of non-verbal scale for measuring fluid intelligence.
Very well put. I think the reasoning behind this is that your general knowledge and vocabulary reflects your ability to learn about your surroundings, which IS really what intelligence is all about, isn’t it? They are assuming of course, that the average test-taker hasn’t been “coached”.