Reported speech: why is the past perfect the right answer here?

Please help me to find out the right answer.

He said that they ______ each other for many years.
a) know
b) have known
c) knew
d) had known

In the book I referred the right answer is d). Anybody can explain why it selected and why the answer is not a)?

Solution 1:

Your book is not wrong, but it is not right, either. Every one of the answers except (a) may be acceptable in some contexts; and I'm not sure that an appropriate context for (a) is impossible.

b. I asked Brian this morning about Harry. He said that they have known each other for years, and anything he says can be relied on.

c. I asked Harry about Tom. He said that they knew each other for years back in the 70s and 80s, then lost touch until 98 when they ran into each other at LaGuardia.

d. I asked Tom about how Kevin came to the firm. He said that they had known each other for years when he founded the firm, and Kevin was the first person he asked to join.