Mercury, sulfur, and salt...

L knew the answer to his riddle... he knew it well, and he desperately wanted to be recognized for it. It had been a great deal of hard work, and to let Asgard down would do nothing for his standing. However, gaining the trust of Light... he'd had an important decision to make.

Mercury, sulfur, and salt.

Approaching the Sphinx and keeping his head bowed, L set an apple on the floor in front of him, and then quickly walked away before Light could see him there.

However...

For himself, for Asgard, for Blythe, L had been unable to throw the Trial entirely. He had removed the core of the apple from the bottom, and carefully replaced it with a sulfur crystal, a bit of salt, and a trickle of mercury from a broken thermometer, all tied together in a misshapen but even globe. The symbol for man's temptation and sin enveloped his presentation to the Sphinx and disguised it from any who might see it, but at the core was Prima Materia, pure and true as the eternal soul of humanity.

With no way to examine the apple closely, Light would have no possible way of discovering what L had done. Neither would a member of Asgard, if indeed the deception would suffice to solve the riddle. L would maintain that he had failed and accept the shame, but in the grand scheme of things, he wanted to have his cake and eat it, too. He wanted to take Light's life and feel like he'd atoned for the sins of his own which had hurt Asgard.