Generally we think Krishna enlightened Brahmā, and that’s a fine basic summary of what happened. But here is a little more detail.
First of all, Brahmā was confused – being born into solitude and universal darkness with no one to teach or guide him. Then he heard a sound, “tapa.” He thought is was his own thoughts, but then he heard it again, “tapa.”
“There is no one else around, so it must be a communication from a greater being trying to help me.” He thought. He concentrated on the sound, and intuited the meaning as “concentrate with discipline.” So he sat to do so.
Who produced that sound?
Generally we think it was the Puruṣa, Garbodakaśayī Viṣṇu. But it may have been someone else on behalf of the Puruṣa. It may have been the primordial divinity of speech/intellect, divya-sarasvatī. Śrīmad Bhāgavatam 2.4.22 says, pracoditā yena purā sarasvatī vitanvatājasya satīḿ smṛtiḿ hṛdi. “In the beginning, Saravatī enlightens the unborn Brahmā’s heart with faithful recollection of knowledge.”
The Brahma Samhita (particularly 5.24-28) elaborates on this and explains that it was in fact “Divya Sarasvatī” (an expansion of Śrīmatī Rādhārāṇī) who came to Brahmā in the universal darkness – perhaps just after he had heard “tapa” and was attempting follow the directive by meditating. She gave him a kāma-gayatrī mantra (kṛṣṇāya govindāya gopī-jana-vallabhāya). By elaborately meditating on this mantra, Brahmā realized it’s source, the flute of Śrī Kṛṣṇa. The 28th verse implies that at this point he had darśan of a mādhurya imbued Hari in Vaikuntha, surrounded by Lakṣmī’s just as he had meditated. The Bhāgavatam (Canto Two, Chapter Nine) describes this quite elaborately, including the concise four verses of instruction he received from Hari (which is famous as the four-verse-Bhāgavatam).
So it appears that Śrīmatī Rādhārāṇī, expanded in the form of primordial Sarasvatī, helped Brahmā attain darśan with Hari, by which he was completely enlightened.