The correct answer would be completely.

The basic rule for forming adverbs is that the suffix -ly is added to the end of the adjective (beautiful – beautifully, wise – wisely, divine – divinely).

In this case, the adjective is “complete”. As the “l” in the suffix -ly is a consonant, there is no need to skip the “e” at the end (it is usually skipped when we glue a suffix beginning with an “i”, for example completing).

Therefore, the spelling completly is incorrect.

In other examples of the possible doubts when forming adverbs, if the adjective has two syllables and ends in -y, then we need to replace the final -y with -ily (happy – happily, hungry – hungrily, lazy – lazily). If the adjective ends with a consonant followed by -le, we replace the final -e with -y (terrible – terribly, comfortable – comfortably, incredible – incredibly).