The target mean strength $f_{cm}$ for concrete mix design obtained from the characteristic strength $f_{ck}$ and standard deviation $\sigma$, as defined in $IS:456-2000$, is

1. $f_{ck}+1.35 \sigma$
2. $f_{ck}+1.45 \sigma$
3. $f_{ck}+1.55 \sigma$
4. $f_{ck}+1.65 \sigma$