From the given options select the one that rightly explains the given expression:

To be above board

To be honest in any business deal

The idiom "to be above board" most clearly explains to be honest in any business deal. The idiom comes from the literal meaning of being above a board, which is to be in a position where you can be seen and cannot be hiding anything. In the same way, when someone is said to be "above board," they are honest and fair in their dealings with others.

The other options are not as accurate. To have a good height means to be tall. To have a big black board means to have a large chalkboard. To become boastful means to start bragging about yourself.