The correct answer is option 3, (Al_2X_3).

To understand why, let`s first break down the given formula for the sodium salt of an anion X, which is (Na_2X). This formula tells us that there are two sodium ions (Na^+) combined with one X ion.

Now, when we want to find the formula for the aluminum salt, we need to consider the charge of the aluminum ion. Aluminum typically has a charge of 3+ (Al^3+). Since the X ion was in the sodium salt formula as (Na_2X), it carries a charge of 1-.

To balance the charges, we need three X ions to cancel out the +3 charge of the aluminum ion. Therefore, the formula for the aluminum salt becomes (Al_2X_3), where there are two aluminum ions combined with three X ions.

Option 1, AlX, is incorrect because it suggests only one X ion, which does not balance the charge of the aluminum ion.

Option 2, (AlX_3), is incorrect because it suggests three X ions but only one aluminum ion, which again does not balance the charges.

Option 4, (Al_3X_2),