Witch prediction is best supported by a detail in the excerpt

The answer is indeed submitting.

Explanation:

When we add -ing to verbs, we must pay attention to a few rules. In general, most verbs won't suffer any changes. However, in some cases, we will need to remove a letter, or even to add one. Study the examples below:

do -- doing

cry -- crying

dance -- dancing

lie -- lying

stop -- stopping

submit -- submitting

Notice that we doubled the last letter of the verb "submit" when we added -ing. That was in obedience to the following rule: When a verb has two syllables, being the last syllable the stressed one and ending in consonant + vowel + consonant, we must double the last letter to add the -ing. Other examples would be:

refer -- referring

prefer -- preferring