Which among the given acids is the strongest

a) Cl-CH2-COOH
b) Br-CH2COOH
c) O2N-CH2COOH
d) NC-CH2COOH

2-Nitroethanoic acid is the strongest out of these. Remember that -NO2 group has the maximum -I effect ie ability to withdraw electrons towards itself thereby making the hydrogens of -COOH group strongly delta positive.
Next in the order will be 2-cyanoethanoic acid because its -I effect is lesser than -NO2
then 2-chloroethanoic acid where chlorine is an even less electron withdrawing element and finally the weakest acid is 2-bromoethanoic acid with Br exhibiting the lowest electron withdrawing tendencies out of these groups.

( a) O2 N- CH2 -COOH NITRO -ACETIC ACID
reason----------------- due to - I- effect of NO2 [ -- ve - inductive effect of NITRO gp ]
electron withdrawing effect ( - ve I -effect )---------------------------------------… ORDER OF DECREASING -ve I effect
NO2>F>COOH>Cl>Br>I> OH > OR