Only consonants. And with no doubt some hidden vowels. There are no words without vowels or maybe there are some? If so, then with no doubt they are not many.
But at first it must be checked if there is any consonant hidden, too. There can be a letter which has not been written, so it is but as if it is not. Readable but invisible (or invisible but readable – like these hidden vowels – unlike many letters that are visible but not readable).
It is possible that these letters are main, most important, that's why they are visible, while less important consonants are not visible, have been hidden due to their unimportance. But why would they be less important? Why would n in SPRinT be less important than other consonants? This is stupid and unreasonable, even more than hiding vowels. Unless we have to do with a language where consonants are in overwhelming majority – there are such languages and they despise and disdain vowels, consider them not worthy to be written if everybody knows where they should be for their position is determined so clearly by the context… And there is no context here. Only these four letters whirling in the blank space… Because in the case of a language where vowels are dominant, consonants could be hidden as not worthy to be written (then in Land of Long White Cloud its name would be written AOEAOA – having omitted two consonants; fortunately writing was unknown there). However here the language seems to have consonants and vowels in balance – more or less – if the consonants are dominant, then only slightly, so hiding randomly chosen consonants would transform reading into solving riddles and puns. Of course SPRT is a pun, but to solve it one needs only to chose just one possibility out of a few. One doesn't need to be wise to do this – it's enough to be SMRT.

SPiRiT Spirit in such a light? No.

SPRouT If this is also an image of a sprout than it is the most strange sprout in the world.

SPuRT A verb? Definitely it should be a noun. Everywhere around are but nouns… But is there any real difference between a verb and a noun? Thing and phenomena not only are – they also are occurring.

PaRTa Here? Now? No. Sparta already was. They say history like to repeat. But they also say that nothing happens twice.

SPoRT Of course! And this whirling arrangement. Windmill wing pattern. Prompting a missing o. And the shadow of a context – the nearby stadium.

So, let it be SPORT.

Is solving puns and puzzles a sport?