step 1: write out the formular for the sum of and Arithmetic series
[tex]\begin{gathered} S_n=\frac{n}{2}(2a+\text{ (n- 1)d)} \\ \text{where} \\ \text{first term a=-10} \\ \text{ n=6} \\ \\ S_6=-285 \end{gathered}[/tex]Step 2: substitute the values of a,n and S6 in the equation above
[tex]\begin{gathered} -285_{}=\frac{6}{2}(2\times(-10)\text{ +(6 - 1)d} \\ -285=3(-20+5d) \\ -285=-60+15d \\ \\ 15d=-285+60 \\ 15d=-225 \\ d=\frac{-225}{15} \\ d=-15 \end{gathered}[/tex]Hence the common difference d of the Arithmetic series is d=-15
Therefore the correct answer is option B
