Let's evaluate the expression \((X \cap Y) \cup X\) where \( X = \{1, 2, 3, 4\} \) and \( Y = \{3, 5, 6\} \).

1. Find \( X \cap Y \) (the intersection of X and Y):
- The intersection includes elements common to both sets.
- \( X \cap Y = \{3\} \)

2. Find \((X \cap Y) \cup X\) (the union of \( X \cap Y \) and X):
- The union includes all elements from both sets, with duplicates removed.
- \( (X \cap Y) \cup X = \{3\} \cup \{1, 2, 3, 4\} \)
- Combining these, we get \( \{1, 2, 3, 4\} \)

Thus, the elements of \((X \cap Y) \cup X\) are A. \{1, 2, 3, 4\}.