a) The p-value of a test is the probability that the null hypothesis is true.

False. 

b) A difference in expression which is highly signifcant must be biologically relevant.

False: could be some kind of "clock"

c) Large samples are bad because small differences can be signifcant.

False. Having a more significant result is not a bad thing

d) A difference in expression which is signifcant can still be due to chance.

True, but very unlikly.