Answer:
b. two-independent sample t test
Explanation:
the question stated that the variance is unknown, and standard deviation is paramount for a t test calculation.
the t-value is determined from the relationship
t = (x−µ) s
where x is the mean value of a sample, µ is the mean value of the population from which the sample is drawn,
s is the standard deviation of the sample and
N is the number of independent observations in the sample.
therefore, for independent sample t test, the variance can be calculated from the values of the two mean given with the sample size.