Answer:
appeal to novelty.
Explanation:
An appeal to novelty can be defined as a fallacy that reasons that an idea is superior to others because it is modern and new. The other name for this fallacy is an appeal to new.
The word 'novelty' is referred to define something new or unused. Thus, the reasoning of an appeal to novelty is based on the fallacy that new is better, thus, superior.
The given statement is an example of an appeal to novelty because the reasoning is based on that one should try the new dietary plan to lose weight. The statement 'newer is always better' hints to the appeal that it is a novelty appeal.
So, the correct answer is an appeal to novelty.