What is the correct formula?

a) Base Life Points + Class Bonus Life Points + Life Points Multiple X Multiple Damage Resistance

OR

b) Base Life Points X Multiple Damage Resistance + Class Bonus Life Points + Life Points Multiple

The Correct forumula is (I believe):

Base Life Points + Class X Level + DP X Resistance Multiple

Actually Damage resistance multiple creatures don't use their class Life Points Multiples, since the "Damage Resistance" multiple is much more efficient way of spending their DP.

In the Core Exxet (in the creature creation chapter under Damage Resistance Multiple) it's specified also with an example that Class Bonus Life Points are multiplied.

A creature with Damage Resistance Multiple x5 and a Class that grants 20 LP per level would gain 100 LP per Class level.

To that amount you then add DP*DRM.

So it's Base LP + Class LP Bonus*Class Level*DRM + DP*DRM

Keep in mind that a Class Life Points Bonus is different from the Class Life Points Multiplier. The bonus is a passive, free amount of LP gained per level.

Also, every time you need to apply a bonus to such a creature's LP total, that amount would be multplied too (from spells, or other powers).

A spell granting 100 extra LP would give 500, unless the specific power says something different.