Choice 'd' represents a risk concentration due to excessive exposure to a single country, even though spread across different industries as the risk factors (economy, exchange rate, interest rate, political risk etc) are the same for all companies in the country.
Choice 'a' represents a risk concentration because even though the risk factors are different, they are highly correlated and therefore effectively behave as one. These undetected correlations proved to be fatal to many financial institutions during the credit crisis.
Choice 'b' represents a risk concentration as was borne out by the recent credit crisis. Large banks had to take over the obligations of SIVs they had created, even though the SIVs were separate legal entities with no legally enforceable recourse to the originating bank. This had to be done for moral and reputational reasons, and banks had to absorb the losses of these supposedly separate vehicles.
Choice 'c' does not represent a risk concentration, in fact it is not a risk at all because it refers to collateral held, even though the collateral may have been provided by the same counterparty. In this case the risk is to the party providing the collateral (in case the party holding the collateral rehypothecates or sells the collateral and is unable to return it).
Therefore Choice 'c' is the correct answer.
The BCBS document on stress testing provides a very nice articulation of risk concentration, and the relevant text from that document is produced verbatim here: [Risk concentration] may arise along different dimensions: single name concentrations; concentrations in regions or industries; concentrations in single risk factors; concentrations that are based on correlated risk factors that reflect subtler or more situation-specific factors, such as previously undetected correlations between market and credit risks, as well as between those risks and liquidity risk; concentrations in indirect exposures via posted collateral or hedge positions; concentrations in off-balance sheet exposure, contingent exposure, non-contractual obligations due to reputational reasons.
