The question asks for the word opposite in meaning to the underlined word 'disingenuous'.
The word disingenuous means not straightforward or candid; giving a false appearance of having knowledge or innocence. It implies a lack of sincerity or honesty, often subtle.
We need to find the word among the options that means the opposite of 'disingenuous'. Let's examine the options:
Comparing the meanings, 'scrupulous' stands out as the opposite of 'disingenuous'. While 'disingenuous' describes someone lacking sincerity and honesty (often in a calculating way), 'scrupulous' describes someone who is highly principled, thorough, and honest.
The other options (duplicitous, mendacious, perfidious) are all synonyms of dishonesty and are therefore unsuitable as antonyms for 'disingenuous'.
Therefore, the correct antonym is scrupulous.