Let us denote the ith term of the sequence by T(i).

Let S(j) be the jth prime number, so that S(1) = 2, S(2) = 3,
S(3) = 5, S(4) = 7, ...and so on.

Then, in terms of the given sequence, we observe that:

T(S(j)) = j, and:
T(S(i1)*S(i2).....S(ip)) = i1*i2*i3.....*ip, with i1< i2< .....< ip,  --------- (i)

We are required to find the 15th term of the given sequence.

Now, 15 = 3*5, so that: (S(i1), S(i2)) = (3, 5), giving: (i1, i2) = (2,3), and accordingly:

T(3*5) = 2*3 = 6

Consequently, the required value corresponding to the mark of interrogation is 6.