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A unit of marines and a unit of ork boyz are locked in combat, at the start of the ork players turn. All marines are in base to base with a ork boy.
The ork player assaults the unit of marines (already locked in combat) with a unit of nobz.
Who can the marines direct thier attacks against?
A. only the boyz
B. combination of the boyz and nobz

The two bullet points on page 41 of the rulebook deal with this i assume.
I think its A, and i play with 2 groups of people, one group says its A and the other B.

Basically you can attack whatever units you are in Base to base with.

So it is B, the marines can attack the boyz and or the nobz.
The Errata clears it up:

Page 41 – Multiple Combats, Attacking.
A third bullet point should be added, as follows:
• Models that at the beginning of the combat (before
any model attacked) were engaged with more than
one enemy unit, but were in base contact with just one
of the enemy units, must attack that unit.

Page 41 – Multiple Combats, Attacking.
In all three bullet points, the word ‘combat’ will be
changed to ‘round of combat’

This is one of the most frequently misunderstood rules in 5th edition.

You determine who can fight at the start of the round of combat, not the start of the assault phase.

So after all assaulting models are moved, and all Defender reactions are made, you move to the first combat to be fought and determine who is engaged with whom.

So in your example, marines in base contact can direct their attacks against the unit or units with which they are in contact. Marines not in base contact can direct their attacks against any unit with which another marine within 2" is in base contact.

How long those units were in base contact is completely irrelevant. It plays no part in determining who can fight, and who they can attack.