Solve this question

Dear Student 
Solution to your asked query is provided in tabular form:
 

$\theta$	Cos$\theta$	W = F.s cos $\theta$	Conclusion
0o	Cos 0o = 1	W = F.s	$$\begin{gathered} when \theta =0 \\ then value of \cos 0^o is 1 \\ therefore workdone = force\times displacement \times \cos 0^o \\ W =F.s.1 =F.s \\ this implies that workdone is positive. \end{gathered}$$
90o	Cos 90o = 0	W = 0	$$\begin{gathered} when \theta = {90}^o then the value of \cos \theta = 0 \\ then workdone is also equals to 0 because \\ Workdone( W) = Force\left(f\right) \times displacement\left(s\right) \times c\mathrm{os}\theta \\ as \cos {90}^o =0 \\ therefore W = 0 \end{gathered}$$
180o	Cos 180o = - 1	W = - F.s	$$\begin{gathered} when \theta ={180}^o \\ then value of\cos {180}^0 =-1 \\ therefore W =F\times s\times \cos \theta \\ W =F\times s\times \cos {180}^0 \\ W=F\times s\times (-1) \\ W=-F.s \\ this implies that workdone is negative \end{gathered}$$

Hope this information clears your doubt about the topic.
Regards