a. "The planning graph data structure can be used to give a better heuristic for a planning problem" - This sentence is generally true. Planning graphs are used in heuristic search algorithms for planning problems to estimate the distance to the goal state.
b. "Dropping negative effects from every action schema in a planning problem results in a relaxed problem" - This sentence is true. In the context of planning problems, creating a relaxed problem involves removing negative effects from action schemas, leading to a simplified version of the original problem.
Therefore, the correct answer is:
Both sentence a and sentence b are true.