3. Channels of distribution
usually require longer-term planning than other marketing mix elements
because channel decisions are more difficult to change quickly.
True. Channel decisions take a long time to plan, because they involve selecting and negotiating with other companies which have sometimes conflicting objectives.
that include middlemen result in higher distribution costs than for channels
Wrong... Remember our myth #1 on middlemen (the so-called "parasites"): because of their specialization, which results in more efficient operations and a better knowledge of the market, costs are usually lower. "Usually": because it depends on the type of product, such as high value goods where the number of customers is limited.
usually do not involve conflicts as long as each channel member has profit
as a goal.
Every company has profit as a goal, that does not preclude that conflicts may appear, especially if one party feels that their margins are too low due to the other party. Conflicts may also arise for non-monetary reasons.
d) should be designed to increase discrepancies of quantity between producers and consumers.
Eh? I though it was designed to decrease discrepancies, but then again, who am I to say...
none of the above.
Well, obviously not, since answer a) is correct.