By the 1830s the cherokee tribe had developed a/an a. strong leadership. b. written language. c. treaty with the government. d. army.

A timeline of the historical "Cherokee Removal" will help us draw an answer later on.

1802- the federal government of the United States promised to extinguish Native American titles to internal Georgia lands in return for the state's formal cession (the act of surrendering or relinquishing possession or control over) of its unincorporated western claim

1815- The US government established a Cherokee Reservation in the district of Arkansas of the Missouri Territory and convinced the Cherokees to voluntarily move to the said area

More treaties were signed in 1817. Despite the challenges,  they were able to give birth to the Cherokee Nation. 

1822- the Cherokee Supreme Court was founded.

They adapted a written constitution by 1827 creating a three-branched government consisting of a legislative, executive and judiciary body.

To cut short in 1830s, the United States Congress enacted the Indian Removal Act to strengthen the treaties and forcibly free up title to the sought over state lands. During this time, one third of the remaining Native Americans left  the area voluntarily especially because the act was being enforced voluntarily by government troops and the Georgia military men.

During this time also, the Constitution of the reunited Cherokee Nation was ratified at Oklahoma at the conclusion of the removal.

Therefore, the answer is letter A. STRONG LEADERSHIP because they were able to unite and to create their own nation and constitution.