Question

Harmonies built on the dominant (fifth scale degree) have a tendency toward the _____.

Answer 1

The word to fill the blank space is "Tonic".
Complete sentence will be as below:

Harmonies built on the dominant (fifth scale degree) have a tendency toward the tonic. 

In musical context, scale degree means the position of a note on a scale relative to the tonic, the main note of the scale and from this each octave is assumed to begin.