Select the word from the list that best fits the definition

mi abuelo fue profesor de matemáticas por cincuenta años.

As I saw on the comments, you said the options are:

ser-yo

ser tu

ser-el/ell/ud

ser-nosostros

ser-urds./ellos

ir-yo

ir-tu

ir-el/ella/ud/

ir-nosostros

ir-uds,/ellos

The correct answer is ser-el/ell/ud .

Explanation:

The verb ser (to be) in this case is conjugated in the past tense, where it becomes FUE.

The verb Fue (was)  indicates something that a person was in a certain period of time in the past, in this case for 50 years.

Now, we have pronouns that accompany this verb, and the one that corresponds to "my grandfather" is the pronoun él.

Given this information, the correct answer is  ser-el/ell/ud .

i would sacrifice nothing unless it were a large problem. hope i answered your question correctly : )