Final answer:
The term 'endorse' best emphasizes that street art is a legitimate art form, conveying support and legitimacy.
Step-by-step explanation:
The writer wants to emphasize that street art is a legitimate art form, and to best accomplish this goal the most suitable choice would be endorse. The term endorse implies support and public approval which are strong indicators of legitimacy, and it highlights the positive stance toward street art. By using endorse, the writer clearly positions street art as a credible and recognized form of artistic expression.