Concept:
In drip irrigation, uniform water discharge from emitters is very important.
Step 1: On slopes and uneven terrain, pressure varies along the lateral pipe.
Step 2: If ordinary drippers are used, discharge may be high at some points and low at others.
Step 3: Pressure compensating drippers maintain nearly uniform discharge even when pressure varies.
Step 4: Therefore, they are suitable for slopes and difficult topographic conditions.
Step 5: Hence, the correct answer is online pressure compensating drippers.
\[
\boxed{\text{Online pressure compensating drippers}}
\]