The correct option is A In addition
The use of the adverb 'regardless' distorts the meaning of the sentence. Here, it seems to imply that the bachelor's degree does not hold any value- as if it is not to be considered in the larger scale- when in fact the sentence means to convey that apart from the bachelor's degree (which is important) some dietitians also 'go on to earn master's degrees in a specific subfield'. To convey this 'also' we need to use 'in addition'. So, option B is the correct choice.