I'm having some problems finding the critical value, some help on these problems would be greatly appreciated, thank you!

round to three decimal places if needed

a) for level 99.5% and sample size 6

critical value=?

b) for level 95% and sample size 15

critical value=?

c) for level 80% and sample size 29

critical value=?

d) for level 98% and sample size 12

critical value=?