Answer:
The correct answer is: "tomó" (took).
Step-by-step explanation:
The sentence is indicating an action that already happen as the part "mientras caminábamos" (while we were walking) is in the past continuos tense (that in Spanish is called "copretérito"). This tense is used when referring to an action that took place in an specific moment and didn't last long (and therefore expressed in the past simple tense) while another action was happening and kept on happening even when the other action ended (and for that we use the past continuos to refer it). Having said that, the first verb then has to be in the past simple tense, as the other verb is the one in the past continuous.
Now, the sentence is referring to "Él" that means He in English, which is the third person singular. So the correct answer is option b. tomó because is conjugated in the past simple for the third person singular.
Option a. tomé is the past simple tense for the first person singular; option c. tomasteis is the past simple tense for the second person singular; and option d. tomaron is the past simple tense for the third person plural.