The lower and upper specification limits for a toothpaste filling machine are $10\mathrm{.}50$ and $12\mathrm{.}00$ ounces, respectively. The standard deviation is $0\mathrm{.}06.$ The process capability index of the machine will be:

a$.$ more than $5.$

b$.$ less than or equal to $3.$

c$.$ more than $4$ but less than or equal to $5.$

d$.$ more than $3$ but less than or equal to $4.$