Final answer:
The incorrect shift in verb mood is found in option a, where a subjunctive mood 'If he were alive' shifts to an indicative mood 'you should speak', which should instead maintain the hypothetical or conditional mood. The correct option is a.
Step-by-step explanation:
The sentence with an incorrect shift in verb mood is option a: If he were alive today, then you should speak to him.
In this sentence, the verb mood shifts from the subjunctive mood (were) to the indicative mood (should speak). The verb mood should remain consistent throughout the sentence.
The sentence with an incorrect shift in verb mood is: If he were alive today, then you should speak to him. This sentence begins with a subjunctive mood ('If he were alive'), which is used to discuss hypothetical or non-real situations, and incorrectly shifts to an indicative mood ('you should speak'), which is used for factual statements or real situations.
The correct mood after 'If he were alive today' would maintain the hypothetical or conditional mood, such as 'you would be able to speak to him.'