How do the 16th 19th and 26th amendment change relationship between state governments and federal government

16th amendment allowed for the federal government to collect income tax without giving any to the states from where the income was earned. 
19th amendment prohibited states from denying the right to vote based on gender, granting women the right to vote nationally. 
26th amendment lowered the voting age from 21 to 18 nationally, thus eliminating state laws preventing people from voting based on age. 

4.a depression is the answer

They weaken the south vietnamese army.