I think you're just guessing based on the molar mass.
take the clues one at a time
It is a white powder <probably doesn't contain S since it's white powder right?>
It has the molar mass of 150g/mol
It's melting point is 165 degrees C
<not a salt & probably doesn't contain any metals>
To neutralise half a mole of the substance you require 1 mol of NaOH < take that as meaning it has 2 acid H's per molecule>
Combustion of the substance gives, among others, 4mol of CO2 <take that as meaning 4 C's per molecule>
Optical activity: 0 degrees <take that as meaning you either (1) have a racemic mixture or (2) don't have
an optically active molecule>
Solubility: 1,25kg / Litre of water
It does not contain any nitrogen atoms or halogens
so let's start here.
what molecules have 4 carbons and 2 acid H's? this help? https://en.wikipedia.org/wiki/Dicarboxylic_acid
or some variation of that 4 carbon dicarboxylic acid?
notice the molar mass is 32g/mol short of the 150 you're given?
would 2 oxygen atoms make the difference?
how would you fit 2 additional O atoms in that molecule?
what would it be called?
what are the possible isomers?
what are the possible optical activities of such a molecule?
do any of those match the 0° optical activity you're given?
do any of those have the mp of 165 °C
which isomer has solubility = 1.25 kg/L?
if you're still stuck what are OH groups called? how would you name a dicarboxylic acid molecule with 4 C's and 2 OH groups? get to googling! The answer is in a second wikipedia link which should be relatively easy to find given what I just told you.