When a block exists within another block, the blocks are a) structured b) nested c) sheltered d) illegal

(B) Nested

Explanation:

 In the computer programming language, the nested is also known as inner class. It is defined as when a block are nested inside the one another block and it is basically used when we want to create similar number of variables and also block withing the program. A variable stops to exist toward the finish of the square where it is proclaimed.

It is used as nested function in the programming language, as nested function is the enclosing type of function which is define with another function in the same program.  

top and bottom view

explanation:

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