Before texas could be restored to the union, president johnson insisted that it .a. stop fighting with the northerners b. give land to the african americans. c. cancel war debts on the north. d. make slavery and sucession illegal

I believe  the answer is D.make slavery and sucession illegal

There are two things that president jhonson insisted before texas could be restored to union:
1. He pardon the people who are loyal to the north
2. He required those who want to join the union to sign and accept the Thirteen Amendment of the constitution, which make involuntary slavery/servitude became illegal.