Answer:
B. 
Step-by-step explanation:
we will verify each options
option-A:
we have

we can see that x and 1 are different terms
so, they are not like terms
so, we can not combine or simplify it
so, this is in simplest form
this is FALSE
option-B:
we have

we can see that 2x and x are same type of terms
so, they are like terms
so, we can combine or simplify it

So, this expression is not in simplest form
this is TRUE
option-C:
we have

we can see that x^2 and x are different terms
so, they are not like terms
so, we can not combine or simplify it
so, this is in simplest form
this is FALSE
option-D:
we have

we can see that 5x^3 and x are different terms
so, they are not like terms
so, we can not combine or simplify it
so, this is in simplest form
this is FALSE