D
In case C is to be the fifth presenter, referring to the condition B’s presentation must immediately precede C’s presentation, B’s presentation must be fourth.

C
In any other position, both the conditions can be satisfied except third because the lunch break would separate the presentations of B and C which is not allowed. except third B could be scheduled for any of the positions-First, Second, Fourth or Fifth.

E
In case after D, F has to present his paper, then as per the last condition, D has to present his paper first, also B has to present before C and there should be no lunch break between the presentation of B and C hence C must be scheduled fifth.

C
If F and E are the fifth and sixth speakers respectively that means D is at number one, then B and C should be second and third. Thus A has to be at number four

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