We know,
2NaOH+CO2→Na2CO3+H2O
So 2 mol NaOH absorbs 1 mol of CO2
Here we have,
300 mL of a 2 M NaOH = 600 mmol of NaOH
So, 600 mmol of NaOH will absorb 300 mmol of CO2
So, we got 300 mmol of CO2 by burning 4.3 g of CnH(2n+2)
So applying POAC on carbon we get,
4.314n+2=0.3n
So, n=6