Convert the statement into indirect speech. 

The father will say, "The cars have become costly".

The given statement, "The father will say, "The cars will become costly," is in direct speech format. Here the actual words of the father are quoted as it is. We have to change it into the indirect format as someone reports it. While converting a statement into the indirect format, if the reporting verb is in the present or future format, the tense of the indirect speech will also remain unchanged. Here, the reporting verb is have become, which is in the simple present tense. So, in the indirect form also, it should be "have become". This gives option B as the right answer. Option A is in the past tense, and option C is in the future continuous tense. Option D also does not justify the given statement by pointing to uncertainty.