The function f(x)=ax+b is strictly increasing for all real x, if
a>0
a<0
a=0
a≤0
Explanation for correct option
Given function f(x)=ax+b
⇒f'(x)=a
We know that for strictly increasing function, f'(x)>0.
⇒a>0
Thus, f(x) is strictly increasing for all real x if a>0.
Hence, the correct option is A.