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Only five words start with IO, and three of them are the same chemical term spelled differently.
D takes the third box in three of the five, sixty percent. IODIC, IODID and IODIN are all built on iodine and differ only in the last two boxes, which leaves very little for a guess to do.
IONIC and IOTAS are the two entries most people would produce. I fills the fourth box in four of the five, eighty percent, so only IOTAS breaks that shape, and confirming the fourth letter as anything but I settles the page at once.
C closes two of the five and nothing else takes more than one. Nothing here repeats a letter, so a visible double rules the page out entirely.
It is the most recognisable entry here. IOTAS is the realistic alternative and the two differ in every box after the IO.
All three appear on this list as forms relating to iodine. They differ only in the fourth and fifth boxes, which makes them hard to separate.
The opening needs a vowel pair that English rarely puts at the front. Five entries is everything the language offers at five letters.