Correct option is C
Given series: 344, 348, 357, ?, 398, 434
344 to 348:
Add 22 = 4
344 + 4 = 348
348 to 357:
Add 32 = 9
348+9=357
357 to ?:
Add 42 = 16
357+16=373
373 to 398:
Add 52 = 25
373+25=398
398 to 434:
Add 62 = 36
398+36=434
Correct answer is (c) 373.