Read the passage carefully and answer the following questions.

Aldehydes, carboxylic acids and ketones are widespread in the animal and plant kingdoms. They play a pivotal role in various biochemical processes of life. Their presence in nature adds fragrance and flavor. These compounds are widely used in food products, pharmaceuticals, paints, resins and other important product industries. These compounds are prepared by various laboratory methods, which mainly include oxidation, formylation, acylation and reduction. Due to the polar nature of the carbonyl group in aldehydes and ketones, they can exhibit different reactions like nucleophilic addition. They do exhibit redox and various condensation reactions which lead to the formation of various important compounds. On the other hand, the carboxylic acids are mainly prepared by oxidation and hydrolysis of different compounds. The carboxylic acid consists of a carbonyl group and the hydroxy group (attached to the carbonyl carbon atom). This makes it possible for the carboxylic acid to participate in various chemical reactions which involve cleavage of the C-OH bond and the O-H bond along with the reactions involving the complete -COOH group.

Arrange the following in the increasing order of their $pk_a$, values:

(A) $CF_3-COOH$
(B) $O_2N-CH_2-COOH$
(C) $NC-CH2-COOH$
(D) $H-COOH$

Choose the correct answer from the options given below:

(D), (C), (B), (A)

The correct answer is Option (1) → (D), (C), (B), (A)

Note: The given answer is as per NTA which is apparently wrong. 

If the question truly says “increasing pKₐ”, then the correct order should be: (A), (B), (C), (D) as calculated below:

Approximate pK_a values and reasoning:

• (A) CF₃−COOH (trifluoroacetic acid): pK_a ≈ 0–0.23 (Three highly electronegative F atoms provide very strong inductive electron withdrawal directly attached to the carboxyl carbon, leading to extreme stabilization of the conjugate base and the highest acidity.) 
• (B) O₂N−CH₂−COOH (nitroacetic acid): pK_a ≈ 1.68 (−NO₂ is a strong electron-withdrawing group with powerful inductive and resonance stabilization of the conjugate base, making it more acidic than −CN substituted.)
• (C) NC−CH₂−COOH (cyanoacetic acid): pK_a ≈ 2.45–2.47 (−CN is a moderate electron-withdrawing group via inductive and resonance effects, stabilizing the conjugate base and increasing acidity.)
• (D) H−COOH (formic acid): pK_a ≈ 3.75 (Standard unsubstituted carboxylic acid; no electron-withdrawing substituent on the α-carbon.)

Here the question asks increasing order of pKₐ, so we must remember:

• Lower pKₐ = stronger acid

• Higher pKₐ = weaker acid

   

Strength of electron-withdrawing groups: CF₃ > NO₂ > CN > H

So acidity order is: CF₃–COOH > NO₂–CH₂–COOH > NC–CH₂–COOH > H–COOH

Therefore pKₐ order (increasing) becomes the reverse: (A) < (B) < (C) < (D)

If the question had said “increasing acidity”, then: (D), (C), (B), (A) would have been correct. Since, the answer of NTA is  (D), (C), (B), (A), there appears to be a mismatch between the wording of the question and the marked answer.. The question should have been :"Arrange the following in order of increasing acidity based on their pKₐ values?".