Some states tend to always be democratic and some states tend to only be republican. Other states, called swing states change whether they vote republican or democratic.
Why would candidates want to campaign in swing states?

it is largely a waste of resources to dedicate time and engery into states that are decidedly Republican or Democrat. By focusing on swing states, a candidate can have more of an impact on their election chances by earning electoral college votes in states that are on the fence. It is very unlikely that a Republican Candidate will win a state like California, also, it is very unlikely that a Republican will lose a state like Oklahoma, so they focus on states that can tip either way.

the correct labels are as follows:

a: yellow river

b: himalayan mountains

c: north china plain

d: taklimakan desert