My grandfather has no interest ---- digital technology

The best preposition for this sentence is: in.

Here's why:

• In indicates a lack of enthusiasm or involvement. It suggests your grandfather doesn't find digital technology interesting or engaging.
• With typically refers to using or working with something. While your grandfather might not use digital technology, "in" emphasizes his lack of interest.
• For suggests a purpose or reason. It wouldn't be accurate to say your grandfather has no purpose related to digital technology.
• At is less common for expressing a lack of interest. It's more often used for specific actions or skills.

Therefore, "in" best captures the idea that your grandfather finds digital technology uninteresting.