Let the positive numbers $a,b,c,d$ be in A.P. then $abc, abd, acd, bcd$ are
(a) not in A.P./G.P./H.P.
(b) in A.P.
(C) in G.P.
(d) in H.P.

Hint: Try to obtain the required terms from given terms using properties of sequences.

Since ,it is given that $a,b,c,d$ are in A.P., then $d,c,b,a$ are also in A.P.
A series of terms is known as a H.P. series when the reciprocals of elements are
in arithmetic progression or A.P.
So,
$ \Rightarrow \dfrac{1}{d},\dfrac{1}{c},\dfrac{1}{b},\dfrac{1}{a}$ are in H.P.
Thus, after multiplying the above terms with $abcd$,
We get,
$ \Rightarrow \dfrac{{abcd}}{d},\dfrac{{abcd}}{c},\dfrac{{abcd}}{b},\frac{{abcd}}{a}$ are in H.P.
$ \Rightarrow abc, abd, acd, bcd$ are in H.P.
Hence, the required answer is (d) in H.P.

Note: To solve these types of questions, perform the specific manipulations and obtain the required solution.