Final answer:
Polonius uses a metaphor in advising Reynaldo to tell gossip and lies about Laertes as a strategy to discover the truth about Laertes' behavior, comparing lies to bait used to catch the larger truth.
Step-by-step explanation:
The figure of speech employed by Polonius in the line 'Your bait of falsehood takes this carp of truth' (2.1.62) is a metaphor. Polonius instructs Reynaldo to tell gossip and lies about his son Laertes in order to indirectly gather the truth about Laertes' behavior in France from the reactions and stories of others. The metaphor describes a deceitful strategy of using small lies (bait) to catch a larger truth (carp), thus not employing simile or hyperbole which involve different mechanisms of comparison, nor personification which gives human traits to non-human entities.