Fill in the blanks with the most appropriate option:-

English is difficult not only to read, ………to write.

but also

The most appropriate option to fill in the blanks is but also.

The sentence is about the difficulty of English, both in reading and writing. The coordinating conjunction "but also" is used to connect two independent clauses that are closely related in meaning. It is the most appropriate conjunction to use in this context, as it describes the two aspects of English that are difficult to learn.

The other options are not as appropriate:

• And can be used to connect two independent clauses that are not closely related in meaning. It is not as appropriate as "but also", as it does not convey the same sense of contrast or comparison.
• But can be used to connect two independent clauses that are closely related in meaning, but it does not have the same sense of contrast or comparison as "but also".
• Therefore is used to introduce a conclusion that follows from the previous statement. It is not as appropriate as "but also", as it does not convey the same sense of contrast or comparison.