An infinitive verb in English is one that is preceded by the preposition TO (and in some situations ended by ING)
The statements that apply to the sentence "To try to write certain sentences is difficult." are:
A. The sentence is correctly written: The statement is grammatically accurate.
B. The sentence contains more than one infinitive: As said before an infinitive verb in English is preceded by the preposition TO, therefore there are two infinitive verbs within it.
C. One infinitive is inside another infinitive phrase: The verb "to try" modifies the meaning of the following infinitive phrase "to write certain sentences is difficult."