Answer:
The correct answer is He wanted to catch the Union off its guard.
Explanation:
First option is wrong as Pope had more soldiers than he did.
Second option is also wrong as McClellan was already defeated by Lee and lost his influence in the Union Army.
Third option is not true, as Pope just gained control over the Union Army.
Fourth option is correct as Lee believed that Union army is demoralized and is not expecting another attack by Lee's forces.
