Final answer:
The misspelled word is in sentence (a): 'altared'. The correct spelling of the word should be 'altered', which means to change something.
Step-by-step explanation:
The sentence that contains a misspelled word is (a) the art historian was certain the painting had been altared. The correct spelling should be 'altered', which means changed. The word altared is incorrect in this context as it mistakenly references an altar, which is a table or platform used in religious rituals. This question tests spelling and vocabulary knowledge, key components of English language proficiency.