There has been a . . . . . . . lack of efficiency in all the crucial areas of the working of Public Sector Undertakings.

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.