Step 1: Understand the task.
We must find the synonym of 'nefarious'. A synonym means nearly the same thing.
Step 2: Find the meaning of 'nefarious'.
'Nefarious' means wicked, evil, and deeply immoral. It describes very bad or criminal acts.
Step 3: Find the meaning of 'heinous'.
'Heinous' means shockingly evil, terrible, and wicked. It too is used for very bad deeds.
Step 4: Test the other options.
'Gentle' means kind, 'virtuous' means morally good, and 'magnificent' means grand. All three are positive or mild, so they are opposite to 'nefarious', not the same.
Step 5: Pick the matching word.
Since both 'nefarious' and 'heinous' point to extreme wickedness, 'heinous' is the right synonym.
Step 6: State the conclusion.
The synonym of 'nefarious' is 'heinous'.
\[ \boxed{\text{heinous}} \]