The word 'difficult' is an adjective. Sometimes, adjectives follow the nouns that they modify.
From the options, D is most appropriate because the word simply modifies the compound noun 'English spellings' by succeeding it. Hence option D is correct.
Option A: 'It is difficult' is a clause and does not modify the noun 'spellings'.
Option B: 'Is difficult' begins with a verb 'is', which is singular. The noun 'spellings', however, is plural. The verb and noun must be of the same number (singular or plural).
Option C: In 'be difficult', 'be' is a base verb and requires the preposition 'to' to form a complete meaning. Without it, the phrase is incorrect.
Hence options A, B and C are incorrect.