In the given sentence, the phrase ‘never have’ must be replaced with ‘never have written’. as the sentence is in Present Perfect Tense and needs the third form of verb to complete it. When the effect of an action done in the past is still on the present then we use present perfect tense. Since, in the given sentence, the subject ‘I’ has not written a good letter in the past and is expected not to write a good one in the future, it is evident that present perfect tense will be used. Also, the two distinct situations are shown by the usage of two different forms of the verb. Hence, the first clause is shown in present perfect and the other is shown in simple future. Thus, option (a) is correct.