"Bill Wright" wrote in message
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As has been said though, in the context of the question, where all the
other alternatives simply had variations on right angle joints and no
brace, its the only clear correct answer.

It's the correct answer but it shows that the author has no idea about
basic joinery techniques.

No, all it is saying is that the one with the diagonal bar is much better
than any of the other ones in the question, not that it is the *best*
solution. Providing the cross-bar is securely fastened (eg screws or bolts)
to the vertical and/or horizontal bars, it will resist the tendency for the
rectangle to bend out of shape into a parallelogram. But it places a lot
more reliance on the fixings that if the cross-bar was in compression rather
than tension.

Ideally the question should have shown a couple of rectangular gates, plus
one with a compression bar and one with a tension bar. That would have
tested people's knowledge even better.