What term refers to a combination of two letterforms to create a single unit?

Ligature is the term which refers to combination of two letterforms to create a single unit.

Explanation:

The two letterforms combined into create a single unit make a ligature. Ligature is the term used to explain that two or more characters when put together touch each other. For example, two lowercase f letters are placed together closely, like 'ff'.

Ligatures are generally used to increase the looking of type like the pairs of character or triplets that have features that tends to combine when used together. The ligature creates smoother change or connection between the characters by connecting the crossbars or by removing dots over the i or changing the shape of the characters.

The answer is A. ligature.