If exposure time is increased while keeping other factors constant, which property of the beam increases?

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Increasing exposure time, with all other factors fixed, raises the total number of x-ray photons produced. That means the beam quantity—the amount of radiation in the beam—increases because more photons are emitted and reach the patient and the detector. The energy of each photon, which determines beam quality, stays the same since kVp and filtration aren’t changing, so the beam’s penetrating power isn’t altered. Image contrast is driven mainly by beam energy and tissue interaction, so it doesn’t increase with longer exposure time. The higher photon output also means a higher patient dose. So the property that increases is beam quantity.