Explanation:

In order to determine the Bronsted acidity of the boron hydrides, we need to consider the stability of the conjugate base formed after the loss of a proton (H+). The more stable the conjugate base, the stronger the acid.

B5H9:
B5H9 is a pentaborane compound with 5 boron atoms and 9 hydrogen atoms. After losing a proton, the conjugate base formed is B5H8-. The negative charge is shared among 5 boron atoms, resulting in a less stable structure. Therefore, B5H9 is not the most acidic among the options.

B10H14:
B10H14 is a decaborane compound with 10 boron atoms and 14 hydrogen atoms. After losing a proton, the conjugate base formed is B10H13-. The negative charge is shared among 10 boron atoms, resulting in a more stable structure compared to B5H9. This increased stability makes B10H14 more acidic than B5H9.

B6H10:
B6H10 is a boron hydride with 6 boron atoms and 10 hydrogen atoms. After losing a proton, the conjugate base formed is B6H9-. The negative charge is shared among 6 boron atoms, similar to B10H14. However, B6H10 is less acidic than B10H14 due to the increased steric hindrance caused by the larger boron cluster.

B4H10:
B4H10 is a boron hydride with 4 boron atoms and 10 hydrogen atoms. After losing a proton, the conjugate base formed is B4H9-. The negative charge is shared among 4 boron atoms, resulting in a more stable structure compared to B5H9. However, B4H10 is less acidic than B10H14 and B6H10 due to the decreased number of boron atoms.

Conclusion:
Among the given options, B10H14 is the most acidic boron hydride. This is because the conjugate base formed after the loss of a proton is the most stable among the options, with the negative charge being shared among 10 boron atoms.