In plain words
But that firmness by which one clings to Dharma, Kāma, and Artha, longing for their fruits as the occasion for each arises, is rājasī, Arjuna.
Word for word
But the firmness by which one holds fast to dharma, kāma and artha, O Arjuna, desiring the fruit as each occasion comes, that firmness, O son of Pṛthā, is rājasī.
यया तु धर्मकामार्थान् धृत्या धारयतेऽर्जुनप्रसङ्गेन फलाकाङ्क्षी धृतिः सा पार्थ राजसी
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
यया तु धर्मकामार्थान् धर्मश्च कामश्च अर्थश्च धर्मकामार्थाः तान् धर्मकामार्थान् धृत्या यया धारयते मनसि नित्यमेव कर्तव्यरूपान् अवधारयति हे अर्जुन, प्रसङ्गेन यस्य यस्य धर्मादेः धारणप्रसङ्गः तेन तेन प्रसङ्गेन फलाकाङ्क्षी च भवति यः पुरुषः, तस्य धृतिः या, सा पार्थ, राजसी ॥ ३४ ॥
In plain words
But that steadiness, Arjuna, by which a person clings — always, in the mind, thinking 'this must be done, this must be done' — to duty, to desire, to gain: whatever duty or pleasure or possession happens to pull at him, he grabs onto it out of attachment, and every time he grabs, he's grabbing for the payoff. That kind of steadiness, Partha, belongs to rajas.
Close to the Sanskrit
But that resolve, O Arjuna, by which — duty and desire and wealth, that is duty-desire-wealth — one holds those, duty, desire, and wealth, in the mind, always, in the form 'this must be done', fixes upon them — through attachment: of whichever duty and so forth there is occasion for attachment, by that very attachment one becomes desirous of the fruit — that person whose resolve this is, that resolve, O Partha, is rajasic.