How much will Bob's widow receive in spousal benefits if his Primary Insurance Amount was $2100 and she starts collecting at her FRA?

When Bob's widow begins collecting spousal benefits at her Full Retirement Age (FRA), she is eligible to receive benefits that are based on his Primary Insurance Amount (PIA). The PIA is the benefit amount that an individual would receive if they claim Social Security at their FRA.

In this scenario, since Bob's PIA is $2100, his widow will receive the full amount of that benefit, which is $2100, because she is starting to collect at her FRA. Collecting at this age ensures that she is entitled to the full benefit without any reduction, which is important to note as benefits can be reduced if claimed early.

In contrast, the other amounts listed do not align with the calculation of spousal benefits based on the provided PIA. The widow's benefit will not exceed Bob's PIA, nor will it be under it, given that she is claiming at her FRA. Therefore, the correct amount that she will receive is exactly $2100.