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It is essential to thorough clean/to clean thoroughly/to thoroughly clean the machine after each use.
Choose the best option to make the sentence perfect. I am a bit confused here because I believe each one of them makes the sentence proper. I do not think if there is any rule for this. If yes, please let me know. Suggestions please. Thank you.
Note the difference:
Correct:
It is essential to thoroughly clean the machine.
Incorrect:
It is essential to to clean thoroughly the machine.
The second example can be correctly constructed like this:
It is essential to to clean the machine thoroughly.
Even so, the first example sounds slightly better to me.
This is a difficult question to answer because it is more a question of style than a question of grammar.
The first option is the worst. "Thorough" is an adjective. It is not a suitable modifier for a verb. One way or another, you should use the adverb "thoroughly". As I judge these options, this is the only one with bad grammar.
The last option is a split infinitive. Old-fashioned style guides recommend (or even demand) that you avoid split infinitives. Personally, I find this to be a good option. I cannot speak for whatever style guide you might be required to follow.
The remaining option avoids the split infinitive, but it poses a different stylistic problem. It places the adverb between the infinitive and its object. I have to recommend that you avoid that. I have no idea whether your style guide considers that to be a problem.
The best option is an option that you don't list. Place the adverb after the infinitive's direct object:
It is essential to clean the machine thoroughly after each use.
However, on a multiple choice test, you cannot choose an option which is not provided.
