Explanation: Iron cores of special construction, often ferrites, are designed to concentrate magnetic flux efficiently with minimal energy loss at radio frequencies. 1. **Increase of Q (Quality Factor):** These cores significantly increase the inductance for a given number of turns compared to air-core inductors. If the resistive losses (both winding resistance and core losses, which are minimized in "special construction" cores) do not increase proportionally, the Q factor ($X_L/R$) improves. A higher Q means the inductor is more efficient and stores more energy per cycle relative to the energy lost. 2. **Increase of Gain:** In resonant circuits (like those found in RF transformers or tank circuits), a higher Q leads to a sharper resonance and greater voltage/current multiplication at the resonant frequency. This results in higher gain for the circuit or stage. 3. **Increase of Selectivity:** Selectivity is the ability to discriminate between desired and undesired frequencies. A higher Q factor results in a narrower bandwidth for resonant circuits. This sharper frequency response means the circuit is more selective, passing the intended signal more efficiently while rejecting nearby unwanted signals more effectively. Since these cores enhance Q, and a higher Q directly contributes to both increased gain and improved selectivity, all listed advantages are correct.