What was day of the week on 21-September-1987?

Answer: Option C

Formula : (Date + Month code + No.of years + No.of leap year + Century code)/7
$$\begin{array}{rl}& =\frac{21+6+87+21+0}{7}\\ & =\frac{135}{7}\\ & =2\\ & =\text{Monday}\end{array}$$