The correct answer is C. conspicuous.
A conspicuous lack of efficiency is a noticeable and obvious lack of efficiency. It is a lack of efficiency that is difficult to ignore or overlook. In the context of public sector undertakings, a conspicuous lack of efficiency could be seen in areas such as delays in service delivery, poor quality of service, and high levels of corruption.
The other options are not as accurate or appropriate in this context. A positive lack of efficiency would be a contradiction in terms, as efficiency is generally seen as a positive thing. A surprising lack of efficiency would suggest that the lack of efficiency was unexpected, which is not necessarily the case. A stimulative lack of efficiency would suggest that the lack of efficiency was actually beneficial, which is also not necessarily the case. An insignificant lack of efficiency would suggest that the lack of efficiency was not important or significant, which is also not the case.
In conclusion, the correct answer is C. conspicuous.