Which type of star has the strongest Balmer absorption lines?

A stars, which are classified as type A on the spectral classification scale, exhibit the strongest Balmer absorption lines. These lines are formed by the absorption of specific wavelengths of light by hydrogen atoms in the star's atmosphere.

In the case of A stars, they have effective temperatures ranging from about 7,500 to 10,000 Kelvin. This range is optimal for the excitation and subsequent absorption of hydrogen Balmer lines, which correspond to transitions of electrons in hydrogen from the second energy level to higher levels. The temperature in A stars is high enough to allow for a considerable amount of hydrogen in its ionized state, allowing for a pronounced dark line at those specific wavelengths in the star's spectrum.

Other types of stars, such as B and O stars, have higher temperatures and tend to ionize hydrogen more completely, leading to weaker or even absent Balmer lines. C stars, which represent cooler stars, do not have the same hydrogen absorption features as A stars, resulting in much weaker Balmer lines. Therefore, the strength of the Balmer absorption lines is most prominent in A stars, making this classification the correct answer.