b
\(\mathrm{MSR}=1 \mathrm{~mm}, \mathrm{CSR}=42, \text { pitch }=1 \mathrm{~mm}\)

\(\mathrm{LC}=\frac{\text { pitch }}{\text { No. of } \mathrm{CSD}}=\left(\frac{1}{100}\right)=0.01 \mathrm{~mm}\)

\(\text { Diameter }=\mathrm{MSR}+\mathrm{LC} \times \mathrm{CSD}\)

\(\text { Diameter }=1+(0.01) \times 42 \mathrm{~mm}\)

\(\text { Diameter }=1.42 \mathrm{~mm}=\frac{\mathrm{x}}{50}\)

\(\therefore \mathrm{x}=71\)