What did lincoln mean by saying that "a house divided against itself cannot stand"?
a. a country divided will be easily open to attack.
b. he would not tolerate the secession of the southern states.
c. the country cannot endure half slave and half free.
d. the two party system was detrimental to the nation.

C. The country cannot endure half slave and half free.

C
He said exactly...

If we could first know where we are and whither we are tending, we could better judge what to do and how to do it. We are now far into the fifth year since a policy was initiated with the avowed object and confident promise of putting an end to slavery agitation. Under the operation of that policy, that agitation has not only not ceased but has constantly augmented. In my opinion, it will not cease until a crisis shall have been reached and passed. "A house divided against itself cannot stand." I believe this government cannot endure, permanently, half slave and half free. I do not expect the Union to be dissolved; I do not expect the house to fall; but I do expect it will cease to be divided. It will become all one thing, or all the other. Either the opponents of slavery will arrest the further spread of it and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction, or its advocates will push it forward till it shall become alike lawful in all the states, old as well as new, North as well as South.