It relates to whether the investor will invest if the combined risk premium of the three investments is 50bps or greater than 10 year Treasury premium, which is 1%(premium over the T-Bill).
So adding the three investment premiums:
T-Bill=0%
10 yr callable Bond= 1%+.8%+.9%=2.7%
MBS=.95%
So after doing the weighted RP (1.22%), you check whether the investor is getting 50bp premium over the 1%. 1.22-1.00=.22<.50, so decide to not invest