1. (L ∨ B) ⊃ C
2. (S ∨ D) • L
3. L 2, Simplification (SIMP)
4. L ∨ B 3, Addition (ADD)
5. C 4,1 Modus Ponens (MP)
In words:
Line 3 says since "(S or D) and L" is true, then L is true on its own.
Line 4 says since L is true, "L or B" is also true. Adding B (or any variable) does not affect the truth since we already know L to be true. ADD is a freebie.
Line 5 applies "L or B" true to the conditional statement to show the truth of C follows.