Because C is negative hence the option is B.

Given:

ab>0. This implies both a and b are of same sign
bc < 0. This implies both b and c are opposite sign

Following matrix can be created based on information given:

a b c
- - +
+ + -

clearly from the matrix above both a and c are always opposite signs; meaning their product will be always negative or ac < 0

hence answer quantity B is bigger. Answer choice B

draw a table and you will see the answer:

According to the premise signs of variables are defined as follows:

A (1) - or (2) +

B (1) - or (2) +

C (1) + or (2) -

Obviously multiplication of a*c in either case gives us a negative number, which is less than Zero

a * b > 0
b * c < 0

multiply equations. = a * b^2 * c < 0
b^2 > 0
= > a *c < 0

C is not necessarily negative

but the trick here is that c and a will definitly have opposite signs

ab>0:

a positive and b positive or a negative and b negative

bc<0:

b is negative and c is positive or b ispoisitive and c is negative

since b and a have the same signs, and b and c have opposite signs, then definitely a and c will have opposite signs