I'd go with a) as using the old chart IMHO fits better with all three elements of the error definition according to the handout on TEM I found
Error: Flight crew actions or inactions that:
• lead to a deviation from crew or organisational intentions or expectations;
• reduce safety margins; and
• increase the probability of adverse operational events on the ground and during flight.
Flying towards the CTA arguably reduces safety margins and increases the probability of an adverse event, too, but in this example it would probably be in accordance with the crew's intentions and expectations (which are however wrong because based on the outdated chart).
BTW, given that kind of questions, I can understand why the potential contribution of CRM/TEM etc. to safety is not too enthusiastically valued among many pilots. What a waste.