Introduction:
NP-hard is a classification in the field of computer science that refers to a set of problems that are at least as hard as the hardest problems in the complexity class NP (nondeterministic polynomial time). These problems are difficult to solve efficiently, and finding an optimal solution may require significant computational resources.

Explanation:
To determine which of the given options is not an example of an NP-hard problem, let's analyze each option:

a) Scheduling identical processor:
Scheduling identical processors refers to assigning tasks to a set of identical processors in order to optimize certain criteria such as minimizing the total completion time or maximizing the throughput. This problem is known as the identical parallel machine scheduling problem and is NP-hard. Therefore, option 'a' is an example of an NP-hard problem.

b) Traveling salesman problem:
The traveling salesman problem (TSP) is a classic optimization problem where the goal is to find the shortest possible route that allows a salesman to visit a set of cities and return to the starting city, visiting each city exactly once. It is a well-known NP-hard problem and is often used as a benchmark for testing optimization algorithms. Therefore, option 'b' is an example of an NP-hard problem.

c) Node cover decision problem:
The node cover decision problem involves finding the minimum number of nodes in a graph that can cover all the edges. This problem is known to be NP-hard, as it can be reduced to the vertex cover problem, which is a classic NP-hard problem. Therefore, option 'c' is an example of an NP-hard problem.

d) 2-Coloring of graph:
The 2-coloring of a graph problem involves assigning one of two colors to each vertex of a graph such that no adjacent vertices have the same color. This problem is known as the graph coloring problem and is NP-hard. Therefore, option 'd' is not the correct answer.

Conclusion:
Based on the analysis, option 'd' is not the correct answer as it represents a problem that is NP-hard. The correct answer should be one of the other options (a, b, or c) which are all examples of NP-hard problems.