"i"

The Lord is involved in our every detail, even the minor ones.  As an infinite, eternal law was fulfilled.

A jot is the horizontal mark on the letter "t", and a tittle is the dot above an "i" or a "j".

When writing cursive, these marks are made after the entire word is penned sometimes causing one to overlook their placement.  He is certain, so sure to go over every jot and tittle, as our loving, eternal Heavenly Father.

Alma 34:13-14

13 Therefore, it is expedient that there should be a great and last sacrifice, and then shall there be, or it is expedient there should be, a stop to the shedding of blood; then shall the law of Moses be fulfilled; yea, it shall be all fulfilled, every jot and tittle, and none shall have passed away.

 14 And behold, this is the whole meaning of the law, every whit pointing to that great and last sacrifice; and that great and last sacrifice will be the Son of God, yea, infinite and eternal.