The idea explored in both “Like the Sun” and “The Open Window” is truthfulness. Option D is correct.
In the story “Like the Sun” Narayan explores themes about truth and its possible consequences.
In the case of the story "The Open Window," one of the themes is the fine line in deciphering truth.
“The Open Window” by Saki deals with the case of a man who needs a cure for his nerves and the story becomes a story within a story.