removing ε production It's not that i dont know how to remove εproduction but when complex problem arises i get confused for example

S-->Aa/aaB

A-->a/ε

B-->bbA/ε

CFG without ε production for the above question is

S-->Aa/aaB/a/aa

A-->a

B-->bbA/bb 

here i replaced A,B with ε in all productions to obtain the result

But for questions like this one 

S-->ABCa/bD

A-->Bc/b

B-->b/ε

C-->c/ε

D-->d

how many possabilities will occur in S production
I understood your confusion 

firstly here C,B,A,S all approach ε at one point

so solving in the method you solved previous example put ε one by one in S then consider all combinations like this

S-->ABCa/bD/BCa/ACa/ABa/Aa/Ba/Ca/a

A-->Bc/b/c

B-->b

C-->c

D-->d
firstly in S for (ABCa) put ε in A then B then C later BC,CA,AB,ABC 
so if you follow this fashion you get the answer