Correct Answer - Option 2 : 12.9 m
According to special theory of relativity,
\({L_{shortening}} = \frac{{{L_{Proper}}}}{\gamma }\)
Where \(\gamma = \frac{1}{{\sqrt {1 - {{\left( {\frac{V}{C}} \right)}^2}} }} = \frac{1}{{\sqrt {1 - {{\left( {\frac{{0.7C}}{C}} \right)}^2}} }}\)
\(\gamma = \frac{1}{{\sqrt {1 - 0.49} }} = \frac{1}{{\sqrt {0.51} }} = 1.40\)
\({L_{Shortening}} = \frac{{18.0}}{{1.40}} = 12.86\;m \approx 12.9\;m\)