Inductive effect : Polarisation of s-bond due to the electron donating or withdrawing nature of groups attached is called inductive effect.
e.g. Attachment of Cl an electronegative atom to an ethane molecule causes the carbons to carry a δ+ charge.
Cl ↞

δ+

CH2

↞

δ+

CH3

This induced polarity due to presence of Cl atom is an example of inductive effect.
Electromeric effect : In the presence of an attacking reagent, the π-electron cloud in a multiple bonded compound tends to get polarized. Such an effect is termed as temporary effect and the original condition is restored if the reagent is removed.

(a) The given order of acidity can be explained by –I effect. Cl is an electronegative atom which withdraws electrons from the adjacent carbon. This in turn pulls electrons from the carboxylic group which results in increased acidity. Thus, more the no. of Cl atoms, higher is the acidic strength.
Cl3CCOOH > Cl2CHCOOH > ClCH2COOH
(b) This is an example of +I effect. –CH3 group is an electron donating group and increases the electron density of the adjacent carbon and this transfers the electron density on the –COOH group. As a result, the release of H as H+ becomes difficult and the acidity decreases.
So, if the number of such –CH3 groups is more on the a-carbon, then the acidic strength decreases.