The first ionization energy (in kJ/mol) of Na, Mg, Al and Si respectively are:

🧠 Order Na, Mg, Al, Si in $IE_1$: Na < Al < Mg < Si — the Mg-Al reversal is the key The question lists elements in the order Na, Mg, Al, Si. Note: this is NOT the order of increasing IE — it's the order in which the elements appear in the periodic table. So the answer must put Mg's value (highest) BEFORE Al's value (lower), even though Al has higher Z.

🗺️ Standard $IE_1$ values (kJ/mol) Na ($3s^1$): 496 — Group 1, easiest in this set. Mg ($3s^2$): 737 — filled subshell, harder than Al. Al ($3p^1$): 577 — lone $3p$ electron, easier than Mg. Si ($3p^2$): 786 — smooth rise after the dip at Al.

In the question's order Na, Mg, Al, Si: 496, 737, 577, 786.

That matches option (a).

⚠️ The trap Option (b) gives 496, 577, 737, 786 — the values themselves are right but in the wrong order. Many students arrange the values as a smooth ascending sequence (low to high) and assume that matches the question's element order. But the question lists Mg before Al, while $IE$(Mg) > $IE$(Al). So the second number must be 737 (Mg), not 577 (Al). Always check whether the question is asking for "in IE order" or "in periodic-table order".

$\boxed{\text{Answer: (a)}}$