B) ClCH2COOH > HCOOH > CH3COOH > C2H5COOH

Explanation:
Acidity is determined by the ability of a compound to release a proton (H+) in a solution. The more easily a compound can release a proton, the stronger the acid it is. Acidity is influenced by factors such as electron density, electronegativity, and the stability of the resulting conjugate base.

In this question, we are comparing the acidity of four carboxylic acids: HCOOH (formic acid), CH3COOH (acetic acid), ClCH2COOH (chloroacetic acid), and C2H5COOH (propionic acid).

1. ClCH2COOH (chloroacetic acid):
Chloroacetic acid consists of a carboxyl group (COOH) with a chlorine atom (Cl) attached to the alpha carbon. The electronegativity of chlorine makes the carboxyl group more acidic compared to the other compounds in this list. The presence of the electron-withdrawing chlorine atom destabilizes the carboxylate ion formed after the release of a proton, making it easier for the compound to donate a proton. Therefore, ClCH2COOH is the strongest acid among the given options.

2. HCOOH (formic acid):
Formic acid consists of a carboxyl group (COOH) with a hydrogen atom (H) attached to the alpha carbon. The absence of any electron-withdrawing groups makes formic acid less acidic than chloroacetic acid.

3. CH3COOH (acetic acid):
Acetic acid also consists of a carboxyl group (COOH) with a methyl group (CH3) attached to the alpha carbon. The presence of the electron-donating methyl group stabilizes the carboxylate ion formed after the release of a proton. This stabilization makes it harder for acetic acid to donate a proton compared to formic acid.

4. C2H5COOH (propionic acid):
Propionic acid consists of a carboxyl group (COOH) with an ethyl group (C2H5) attached to the alpha carbon. Similar to acetic acid, the presence of the electron-donating ethyl group stabilizes the carboxylate ion and reduces the acidity of propionic acid.

Therefore, the correct order of acidity is ClCH2COOH > HCOOH > CH3COOH > C2H5COOH.