Step 1: Understand the meaning of ``Optimistic''.
Optimistic means having a positive outlook or expecting good outcomes.
Step 2: Identify the opposite meaning.
The opposite of expecting positive outcomes is expecting negative outcomes.
Step 3: Analyze the options.
Hopeful and Idealistic are similar to optimistic.
Realistic is neutral, not opposite.
Pessimistic means having a negative outlook — opposite of optimistic.
Step 4: Conclusion.
Hence, the correct antonym is Pessimistic.