Hello there!
To find which tuna is a better deal, divide the cost by the number of ounces of tuna you are getting to get the cost per ounce.
$0.90/5 = $0.18 per ounce
$2.40/12 = $0.20 per ounce
Since the 5-ounce can of tuna has a cheaper unit rate price, meaning you are getting a better value, makes this the best option. I hope this was helpful and have a great day! :)