A simple sentence consists of only one clause.
A compound sentence consists of two or more independent clauses which are connected by the coordinated conjunctions.
A complex sentence consists of at least one independent and at least one dependent clause.
A simple sentence can be converted into a complex sentence by expanding a word or phrase into a complex sentence.
Asimple sentence can be converted into acompound sentence by expanding a word or phrase into a clause and by using a coordinating conjunction to connect the clauses.
The correct answer is A)The weather is expected to be good tomorrow.
Option A is correct as the same tense is used (simple present tense) and it is replaced by 'weather'.
The other options are wrong as was , has been, and will be can't be used.