They say the fruit of a well-done action is sattvic and pure; but the fruit of rajas is suffering, and the fruit of tamas is ignorance.
Explanation
Krishna is explaining to Arjuna how the three qualities of nature (guṇas) shape the results of what we do. Sattva (goodness, clarity) brings a pure result, rajas (restless passion) brings suffering, and tamas (inertia, dullness) brings ignorance (ajñāna). Commentators differ on whether "well-done action" (sukṛta) means righteous deeds in general or action done in a sattvic spirit, but all agree the verse links each quality to its typical outcome.