When there's multiple choice, evaluate each choice. Be careful because you have to check the whole range described by each choice. If you find one exception, that choice is invalid. So think about the whole range and try to find an exception. Also note the values don't have to be integer values.
Assuming this is 
A. When the value of b is less than 8, a is negative.
b=7 a= 3.5 -4 = negative
b=7.9 a = #<4 - 4 = negative
b=0 a = 0-4 = negative
b=-1 a = -1/2 -4 negative - and note that as b becomes larger and larger negative a becomes larger and larger negative.
Statement A is true.
Check the others just to make certain.
B. When the value of b is greater than 8, a is negative.
b=100, a=50-4 is positive. NO!
C. When the value of b is less than 8, a is positive. No - we saw that while evaluation A. If b=0, a = 0-4 = negative.
D. When the value of b is greater than 4, a is positive. No. If b=5, a=2.5 - 4 = negative.
Hope the solution helped. Sometimes you need more than a solution. Contact fcabanski@hotmail.com for online, private tutoring, or personalized problem solving (quick for groups of problems.)
(