To identify all subscribers who were sent Job ID 420 but did not click any links, the developer should use a SQL statement that selects subscribers from the _Sent Data View where the Job ID is 420 and excludes those who are found in the _Click Data View. The correct SQL statement is:

SELECT s.SubscriberKey FROM _Sent s LEFT JOIN _Click c ON s.SubscriberKey = c.SubscriberKey AND s.JobID = c.JobID WHERE s.JobID = '420' AND c.SubscriberKey IS NULL

This query performs a left join between the _Sent and _Click data views and filters the results to include only those subscribers who do not have a corresponding click record.