Step 1: Understand the question.
We need the generation of programming languages that is closest to plain human language.
Step 2: Recall the generations.
Languages are grouped by how far they sit from machine code. 1GL is machine language, 2GL is assembly, and 3GL covers high level languages like C and Java.
Step 3: Move higher up.
4GL is a very high level group used for databases and reports. It is easier than 3GL but still uses set commands.
Step 4: Reach the top level.
5GL is tied to Artificial Intelligence and natural language. Here you describe the problem in words close to human speech, and the system works out the steps.
Step 5: Compare with the options.
Among 2GL, 3GL, 4GL, and 5GL, the higher the number, the closer to human language. So 5GL is the nearest.
Step 6: Conclude.
So the answer is option 4, 5GL.
\[ \boxed{\text{5GL}} \]