Answer:
C). She was so excited she started crying and laughing all at once, my mother was less sure.
Step-by-step explanation:
A comma splice is illustrated as the grammatical error as a comma is employed to link two independent clauses with a comma instead of employing conjunction or semicolon.
In the given question, option A is not grammatically correct and contains a 'comma splice' as in this option only two independent clauses 'She was so excited she started crying and laughing all at once' and 'my mother was less sure' are joined with a comma(,) instead of using a conjunction(like but, although, etc.) or semicolon(;). The other options are grammatically correct as they are joined either by employing conjunction(like 'but' and 'and' in option A and B) or by a semicolon (in option D). Thus, option C is the correct answer.