Why is a) more acidic than b) when C is more electron withdrawing than H? Electron withdrawal by a substituent increases acidity...

 

You have the same situation as in your last acidity question.

The methyl group supplies electrons to the carbonyl group more readily than a hydrogen.

 

See also the inductive effect

http://en.wikipedia.org/wiki/Inductive_effect

 

a is more acidic because it easily loses its hydrogen atoms more than b. the tendency to lose electrons increases with decrease in Methyl groups attached to the parent Carbon.

 

a is more acidic because it easily loses its hydrogen atoms more than

It loses a proton, not a hydrogen atom.

b. the tendency to lose electrons{from what?} increases with decrease in Methyl groups attached to the parent Carbon.

Not sure what you mean to say here. (CH3)3CH (three attached methyl groups) forms a much more stable carbonium ion (i.e., loss of electron from carbon) than CH4 (no attached methyl group). In other words, the ease of electron loss from the carbon increases with the number of attached methyl groups.

John