Which one of these statements is CORRECT regarding exchange$-$traded funds $($ETFs$)?$

Question $4$ options:

a$)$

They also have lower turnover of assets than mutual funds and are, as a result, more tax efficient

b$)$

In$-$kind exchanges typically are made in blocks of $10,000$ shares.

c$)$

An ETF that buys all the securities in an index uses representative sampling.

d$)$

Because of their unique structure, ETFs cannot offer currency ETFs.