Firstly lets calculate the mean.

µ = ∑xi / N

 where xi is the value of the ith discrete entity and N is the total entities.

Here, N = 4   ( since we have four marks)

now, µ = (100+79+86+91)/4 = 89

now standard deviation σ = √{(1/N) ∑(xi-µ)^2}

→ σ = √{((100-89)^2 + (79-89)^2 + (86-89)^2 + (91-89)^2)/4}

      = √{(121 + 100 + 9 + 4) /4 }

      = √58.5

      = 7.6485