Concept:For nCr to be defined, n and r must be non-negative integers and r≤n.Explanation:Here, n=10−x and r=x−6.Both n and r must be non-negative:10−x≥0⇒x≤10x−6≥0⇒x≥6Also, r≤n gives:x−6≤10−x⇒2x≤16⇒x≤8Combining: 6≤x≤8Since x must be an integer, x∈{6,7,8}.Answer:Option D: {6,7,8}