Genesis 22:13 (MT) reads:
וַיִּשָּׂ֨א אַבְרָהָ֜ם אֶת־עֵינָ֗יו וַיַּרְא֙ וְהִנֵּה־אַ֔יִל אַחַ֕ר נֶאֱחַ֥ז בַּסְּבַ֖ךְ בְּקַרְנָ֑יו וַיֵּ֤לֶךְ אַבְרָהָם֙ וַיִּקַּ֣ח אֶת־הָאַ֔יִל וַיַּעֲלֵ֥הוּ לְעֹלָ֖ה תַּ֥חַת בְּנֽוֹ׃
And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in the thicket by his horns. And Abraham went and took the ram, and offered him up for a burnt-offering in the stead of his son. (JPS)
The bolded word, אַחַ֕ר, is a source of debate in this verse.
There are three Hebrew words that look similar to each other, and could all potentially fit in as suitable variants for this verse.
A second possibility is the word אחר 'achêr or akh-air'. This differs from the Masoretic reading by a small change in vowelization of the Hebrew letter heth in this word, but does not require an emendation of the consonantal text. It can be translated as H312 or H313 .
A third possibility is the word אחד 'âchad or aw-khad', which may be supported by various ancient translations. This differs from the Masoretic reading in the final Hebrew letter, which would be changed from a resh to a daleth, and the vowelization may or may not be changed at all. It would be translated as H258 or H259.
Which of these is most likely to be the original reading, and what factors motivate your answer?