Find the unit digit in the product : ( 4387 )^245 × ( 621 )^72

Answer: Option D

$$\begin{array}{c}\text{ unit place}\text{hfill}\\ \left[\begin{array}{c}{7}^1\to 7\to 7\text{hfill}\\ 7^2\to 49\to 9\text{hfill}\\ 7^3\to 343\to 3\text{hfill}\\ 7^4\to 2401\to 1\text{hfill}\end{array}\right]\text{cycle }\text{hfill}\end{array}$$
$$\begin{array}{rl}& {\left(4387\right)}^{245}\times {\left(621\right)}^{72}\\ & \stackrel{\downarrow }{{\left(7\right)}^{245}}\times {\left(1\right)}^{72}\\ & \stackrel{\downarrow }{{\left(7\right)}^{4\times 61+1}}\times 1\\ & \stackrel{\downarrow }{{\left(7\right)}^{4\times 61}\times 7}\times 1\\ & \stackrel{\downarrow }{{\left(1\right)}^{61}\times 7}\times 1\end{array}$$
unit digit = 7