Stable ∞-categories

Which of the following is the definition of a stable ∞-category?

In $\mathcal{D}(\Z)$, let $f : \Z \to \Z$ be multiplication by $6$, with both copies of $\Z$ in degree $0$. How many elements does $H_0(\mathrm{cofib}\, f)$ have?

Pointed spaces form a pointed ∞-category with all limits and colimits. Which computation shows directly that it is not stable?

In the proof that a pushout loop square forces $X \cong 0$ in an ordinary category, we show $\mathrm{id}_X = 0$. Which property of the pushout is used, and how?

In stable_one_category_trivial, which hypothesis correctly formalises 'every fibre sequence is a cofibre sequence' for an ordinary category C with a zero object?

In the worked example, work over the field $k = \mathbb{F}_5$. How many morphisms of triangles $(a, b, c)$ from $T = (k \to 0 \to k[1] \to k[1])$ to $T' = (0 \to k[1] \to k[1] \to 0)$ extend the square $(a, b) = (0, 0)$?

Why is $[X, Y] = \pi_0 \mathrm{Map}(X, Y)$ an abelian group in a stable ∞-category?

Which statements about enhancements of triangulated categories are true? (Choose all that apply.)

A linear map $f : \mathbb{Q}^m \to \mathbb{Q}^n$ is given by an $n \times m$ matrix A (a list of n rows). Viewed as a map of complexes in degree 0, its cofibre is the complex $\mathbb{Q}^m \xrightarrow{A} \mathbb{Q}^n$ in degrees 1 and 0. Using the provided rank, write cone_homology(A, m, n) returning (dim H_1, dim H_0) of the cofibre.